The C++ language has gone through a major transformation in the past decade with the development and release of C++11 and then later with its newer versions C++14 and C++17. These new standards have introduced new concepts, simplified or extended existing syntax and semantics, and overall transformed the way we write code. C++11 looks like a new language, and code written using the new standards is called modern C++ code.

Automatic type deduction is one of the most important and widely used features in modern C++. The new C++ standards have made it possible to use auto as a placeholder for types in various contexts and let the compiler deduce the actual type. In C++11, auto can be used for declaring local variables and for the return type of a function with a trailing return type. In C++14, auto can be used for the return type of a function without specifying a trailing type and for parameter declarations in lambda expressions. Future standard versions are likely to expand the use of auto to even more cases. The use of auto in these contexts has several important benefits. Developers should be aware of them, and prefer auto whenever possible. An actual term was coined for this by Andrei Alexandrescu and promoted by Herb Sutter--almost always auto (AAA).

Consider using auto as a placeholder for the actual type in the following situations:

• To declare local variables with the form auto name = expression when you do not want to commit to a specific type:

        auto i = 42;          // int 
        auto d = 42.5;        // double 
        auto s = "text";      // char const * 
        auto v = { 1, 2, 3 }; // std::initializer_list<int> 

• To declare local variables with the auto name = type-id { expression } form when you need to commit to a specific type:

        auto b  = new char[10]{ 0 };            // char* 
        auto s1 = std::string {"text"};         // std::string
        auto v1 = std::vector<int> { 1, 2, 3 }; // std::vector<int>
        auto p  = std::make_shared<int>(42);    // std::shared_ptr<int>

• To declare named lambda functions, with the form auto name = lambda-expression, unless the lambda needs to be passed or return to a function:

        auto upper = [](char const c) {return toupper(c); };

• To declare lambda parameters and return values:

        auto add = [](auto const a, auto const b) {return a + b;};

• To declare function return type when you don't want to commit to a specific type:

        template <typename F, typename T> 
        auto apply(F&& f, T value) 
        { 
          return f(value); 
        }

The auto specifier is basically a placeholder for an actual type. When using auto, the compiler deduces the actual type from the following instances:

• From the type of the expression used to initialize a variable, when auto is used to declare variables.
• From the trailing return type or the type of the return expression of a function, when auto is used as a placeholder for the return type of a function.

In some cases, it is necessary to commit to a specific type. For instance, in the preceding example, the compiler deduces the type of s to be char const *. If the intention was to have a std::string, then the type must be specified explicitly. Similarly, the type of v was deduced as std::initializer_list<int>. However, the intention could be to have a std::vector<int>. In such cases, the type must be specified explicitly on the right side of the assignment.

There are some important benefits of using the auto specifier instead of actual types; the following is a list of, perhaps, the most important ones:

• It is not possible to leave a variable uninitialized. This is a common mistake that developers do when declaring variables specifying the actual type, but it is not possible with auto that requires an initialization of the variable in order to deduce the type.
• Using auto ensures that you always use the correct type and that implicit conversion will not occur. Consider the following example where we retrieve the size of a vector to a local variable. In the first case, the type of the variable is int, though the size() method returns size_t. That means an implicit conversion from size_t to int will occur. However, using auto for the type will deduce the correct type, that is, size_t:

        auto v = std::vector<int>{ 1, 2, 3 }; 
        int size1 = v.size();       
        // implicit conversion, possible loss of data 
        auto size2 = v.size(); 
        auto size3 = int{ v.size() };  // error, narrowing conversion

• Using auto promotes good object-oriented practices, such as preferring interfaces over implementations. The lesser the number of types specified the more generic the code is and more open to future changes, which is a fundamental principle of object-oriented programming.
• It means less typing and less concern for actual types that we don't care about anyways. It is very often that even though we explicitly specify the type, we don't actually care about it. A very common case is with iterators, but one can think of many more. When you want to iterate over a range, you don't care about the actual type of the iterator. You are only interested in the iterator itself; so, using auto saves time used for typing possibly long names and helps you focus on actual code and not type names. In the following example, in the first for loop, we explicitly use the type of the iterator. It is a lot of text to type, the long statements can actually make the code less readable, and you also need to know the type name that you actually don't care about. The second loop with the auto specifier looks simpler and saves you from typing and caring about actual types.

        std::map<int, std::string> m; 
        for (std::map<int,std::string>::const_iterator it = m.cbegin();
          it != m.cend(); ++it) 
        { /*...*/ } 

        for (auto it = m.cbegin(); it != m.cend(); ++it)
        { /*...*/ }

• Declaring variables with auto provides a consistent coding style with the type always in the right-hand side. If you allocate objects dynamically, you need to write the type both on the left and right side of the assignment, for example, int p = new int(42). With auto, the type is specified only once on the right side.

However, there are some gotchas when using auto:

• The auto specifier is only a placeholder for the type, not for the const/volatile and references specifiers. If you need a const/volatile and/or reference type, then you need to specify them explicitly. In the following example, foo.get() returns a reference to int; when variable x is initialized from the return value, the type deduced by the compiler is int, and not int&. Therefore, any change to x will not propagate to foo.x_. In order to do so, one should use auto&:

        class foo { 
          int x_; 
        public: 
          foo(int const x = 0) :x_{ x } {} 
          int& get() { return x_; } 
        }; 

        foo f(42); 
        auto x = f.get(); 
        x = 100; 
        std::cout << f.get() << std::endl; // prints 42

• It is not possible to use auto for types that are not moveable:

        auto ai = std::atomic<int>(42); // error

• It is not possible to use auto for multi-word types, such as long long, long double, or struct foo. However, in the first case, the possible workarounds are to use literals or type aliases; as for the second, using struct/class in that form is only supported in C++ for C compatibility and should be avoided anyways:

        auto l1 = long long{ 42 }; // error 
        auto l2 = llong{ 42 };     // OK 
        auto l3 = 42LL;            // OK

• If you use the auto specifier but still need to know the type, you can do so in any IDE by putting the cursor over a variable, for instance. If you leave the IDE, however, that is not possible anymore, and the only way to know the actual type is to deduce it yourself from the initialization expression, which could probably mean searching through the code for function return types.

The auto can be used to specify the return type from a function. In C++11, this requires a trailing return type in the function declaration. In C++14, this has been relaxed, and the type of the return value is deduced by the compiler from the return expression. If there are multiple return values they should have the same type:

    // C++11 
    auto func1(int const i) -> int 
    { return 2*i; } 

    // C++14 
    auto func2(int const i) 
    { return 2*i; }

As mentioned earlier, auto does not retain const/volatile and reference qualifiers. This leads to problems with auto as a placeholder for the return type from a function. To explain this, let us consider the preceding example with foo.get(). This time we have a wrapper function called proxy_get() that takes a reference to a foo, calls get(), and returns the value returned by get(), which is an int&. However, the compiler will deduce the return type of proxy_get() as being int, not int&. Trying to assign that value to an int& fails with an error:

    class foo 
    { 
      int x_; 
    public: 
      foo(int const x = 0) :x_{ x } {} 
      int& get() { return x_; } 
    }; 

    auto proxy_get(foo& f) { return f.get(); } 

    auto f = foo{ 42 }; 
    auto& x = proxy_get(f); // cannot convert from 'int' to 'int &'

To fix this, we need to actually return auto&. However, this is a problem with templates and perfect forwarding the return type without knowing whether that is a value or a reference. The solution to this problem in C++14 is decltype(auto) that will correctly deduce the type:

    decltype(auto) proxy_get(foo& f) { return f.get(); } 
    auto f = foo{ 42 }; 
    decltype(auto) x = proxy_get(f);

The last important case where auto can be used is with lambdas. As of C++14, both lambda return type and lambda parameter types can be auto. Such a lambda is called a generic lambda because the closure type defined by the lambda has a templated call operator. The following shows a generic lambda that takes two auto parameters and returns the result of applying operator+ on the actual types:

    auto ladd = [] (auto const a, auto const b) { return a + b; }; 
    struct 
    { 
       template<typename T, typename U> 
       auto operator () (T const a, U const b) const { return a+b; } 
    } L;

This lambda can be used to add anything for which the operator+ is defined. In the following example, we use the lambda to add two integers and to concatenate to std::string objects (using the C++14 user-defined literal operator ""s):

    auto i = ladd(40, 2);            // 42 
    auto s = ladd("forty"s, "two"s); // "fortytwo"s

• Creating type aliases and alias templates
• Understanding uniform initialization

In C++, it is possible to create synonyms that can be used instead of a type name. This is achieved by creating a typedef declaration. This is useful in several cases, such as creating shorter or more meaningful names for a type or names for function pointers. However, typedef declarations cannot be used with templates to create template type aliases. An std::vector<T>, for instance, is not a type (std::vector<int> is a type), but a sort of family of all types that can be created when the type placeholder T is replaced with an actual type.

In C++11, a type alias is a name for another already declared type, and an alias template is a name for another already declared template. Both of these types of aliases are introduced with a new using syntax.

• Create type aliases with the form using identifier = type-id as in the following examples:

        using byte    = unsigned char; 
        using pbyte   = unsigned char *; 
        using array_t = int[10]; 
        using fn      = void(byte, double); 

        void func(byte b, double d) { /*...*/ } 

        byte b {42}; 
        pbyte pb = new byte[10] {0}; 
        array_t a{0,1,2,3,4,5,6,7,8,9}; 
        fn* f = func;

• Create alias templates with the form template<template-params-list> identifier = type-id as in the following examples:

        template <class T> 
        class custom_allocator { /* ... */}; 

        template <typename T> 
        using vec_t = std::vector<T, custom_allocator<T>>; 

        vec_t<int>           vi; 
        vec_t<std::string>   vs; 

For consistency and readability, you should do the following:

• Not mix typedef and using declarations for creating aliases.
• Use the using syntax to create names of function pointer types.

A typedef declaration introduces a synonym (or an alias in other words) for a type. It does not introduce another type (like a class, struct, union, or enum declaration). Type names introduced with a typedef declaration follow the same hiding rules as identifier names. They can also be redeclared, but only to refer to the same type (therefore, you can have valid multiple typedef declarations that introduce the same type name synonym in a translation unit as long as it is a synonym for the same type). The following are typical examples of typedef declarations:

    typedef unsigned char   byte; 
    typedef unsigned char * pbyte; 
    typedef int             array_t[10]; 
    typedef void(*fn)(byte, double); 

    template<typename T> 
    class foo { 
      typedef T value_type; 
    }; 

    typedef std::vector<int> vint_t;

A type alias declaration is equivalent to a typedef declaration. It can appear in a block scope, class scope, or namespace scope. According to C++11 paragraph 7.1.3.2:

An alias-declaration is, however, more readable and more clear about the actual type that is aliased when it comes to creating aliases for array types and function pointer types. In the examples from the How to do it... section, it is easily understandable that array_t is a name for the type array of 10 integers, and fn is a name for a function type that takes two parameters of type byte and double and returns void. That is also consistent with the syntax for declaring std::function objects (for example, std::function<void(byte, double)> f).

The driving purpose of the new syntax is to define alias templates. These are templates which, when specialized, are equivalent to the result of substituting the template arguments of the alias template for the template parameters in the type-id.

It is important to take note of the following things:

• Alias templates cannot be partially or explicitly specialized.
• Alias templates are never deduced by template argument deduction when deducing a template parameter.
• The type produced when specializing an alias template is not allowed to directly or indirectly make use of its own type.

Brace-initialization is a uniform method for initializing data in C++11. For this reason, it is also called uniform initialization. It is arguably one of the most important features from C++11 that developers should understand and use. It removes previous distinctions between initializing fundamental types, aggregate and non-aggregate types, and arrays and standard containers.

For continuing with this recipe, you need to be familiar with direct initialization that initializes an object from an explicit set of constructor arguments and copy initialization that initializes an object from another object. The following is a simple example of both types of initialization, but for further details, you should see additional resources:

    std::string s1("test");   // direct initialization 
    std::string s2 = "test";  // copy initialization

To uniformly initialize objects regardless of their type, use the brace-initialization form {} that can be used for both direct initialization and copy initialization. When used with brace initialization, these are called direct list and copy list initialization.

    T object {other};   // direct list initialization 
    T object = {other}; // copy list initialization

Examples of uniform initialization are as follows:

• Standard containers:

        std::vector<int> v { 1, 2, 3 };
        std::map<int, std::string> m { {1, "one"}, { 2, "two" }};

• Dynamically allocated arrays:

        int* arr2 = new int[3]{ 1, 2, 3 };    

• Arrays:

        int arr1[3] { 1, 2, 3 }; 

• Built-in types:

        int i { 42 };
        double d { 1.2 };    

• User-defined types:

        class foo
        {
          int a_;
          double b_;
        public:
          foo():a_(0), b_(0) {}
          foo(int a, double b = 0.0):a_(a), b_(b) {}
        }; 

        foo f1{}; 
        foo f2{ 42, 1.2 }; 
        foo f3{ 42 };

• User-defined POD types:

        struct bar { int a_; double b_;};
        bar b{ 42, 1.2 };

Before C++11 objects required different types of initialization based on their type:

• Fundamental types could be initialized using assignment:

        int a = 42; 
        double b = 1.2;

• Class objects could also be initialized using assignment from a single value if they had a conversion constructor (prior to C++11, a constructor with a single parameter was called a conversion constructor):

        class foo 
        { 
          int a_; 
        public: 
          foo(int a):a_(a) {} 
        }; 
        foo f1 = 42;

• Non-aggregate classes could be initialized with parentheses (the functional form) when arguments were provided and only without any parentheses when default initialization was performed (call to the default constructor). In the next example, foo is the structure defined in the How to do it... section:

        foo f1;           // default initialization 
        foo f2(42, 1.2); 
        foo f3(42); 
        foo f4();         // function declaration

• Aggregate and POD types could be initialized with brace-initialization. In the next example, bar is the structure defined in the How to do it... section:

        bar b = {42, 1.2}; 
        int a[] = {1, 2, 3, 4, 5};

Apart from the different methods of initializing the data, there are also some limitations. For instance, the only way to initialize a standard container was to first declare an object and then insert elements into it; vector was an exception because it is possible to assign values from an array that can be prior initialized using aggregate initialization. On the other hand, however, dynamically allocated aggregates could not be initialized directly.

All the examples in the How to do it... section use direct initialization, but copy initialization is also possible with brace-initialization. The two forms, direct and copy initialization, may be equivalent in most cases, but copy initialization is less permissive because it does not consider explicit constructors in its implicit conversion sequence that must produce an object directly from the initializer, whereas direct initialization expects an implicit conversion from the initializer to an argument of the constructor. Dynamically allocated arrays can only be initialized using direct initialization.

Of the classes shown in the preceding examples, foo is the one class that has both a default constructor and a constructor with parameters. To use the default constructor to perform default initialization, we need to use empty braces, that is, {}. To use the constructor with parameters, we need to provide the values for all the arguments in braces {}. Unlike non-aggregate types where default initialization means invoking the default constructor, for aggregate types, default initialization means initializing with zeros.

Initialization of standard containers, such as the vector and the map also shown above, is possible because all standard containers have an additional constructor in C++11 that takes an argument of type std::initializer_list<T>. This is basically a lightweight proxy over an array of elements of type T const. These constructors then initialize the internal data from the values in the initializer list.

The way the initialization using std::initializer_list works is the following:

• The compiler resolves the types of the elements in the initialization list (all elements must have the same type).
• The compiler creates an array with the elements in the initializer list.
• The compiler creates an std::initializer_list<T> object to wrap the previously created array.
• The std::initializer_list<T> object is passed as an argument to the constructor.

An initializer list always takes precedence over other constructors where brace-initialization is used. If such a constructor exists for a class, it will be called when brace-initialization is performed:

    class foo 
    { 
      int a_; 
      int b_; 
    public: 
      foo() :a_(0), b_(0) {} 

      foo(int a, int b = 0) :a_(a), b_(b) {} 
      foo(std::initializer_list<int> l) {} 
    }; 

    foo f{ 1, 2 }; // calls constructor with initializer_list<int>

The precedence rule applies to any function, not just constructors. In the following example, two overloads of the same function exist. Calling the function with an initializer list resolves to a call to the overload with an std::initializer_list:

    void func(int const a, int const b, int const c) 
    { 
      std::cout << a << b << c << std::endl; 
    } 

    void func(std::initializer_list<int> const l) 
    { 
      for (auto const & e : l) 
        std::cout << e << std::endl; 
    } 

    func({ 1,2,3 }); // calls second overload

This, however, has the potential of leading to bugs. Let's take, for example, the vector type. Among the constructors of the vector, there is one that has a single argument representing the initial number of elements to be allocated and another one that has an std::initializer_list as an argument. If the intention is to create a vector with a preallocated size, using the brace-initialization will not work, as the constructor with the std::initializer_list will be the best overload to be called:

    std::vector<int> v {5};

The preceding code does not create a vector with five elements, but a vector with one element with a value 5. To be able to actually create a vector with five elements, initialization with the parentheses form must be used:

    std::vector<int> v (5);

Another thing to note is that brace-initialization does not allow narrowing conversion. According to the C++ standard (refer to paragraph 8.5.4 of the standard), a narrowing conversion is an implicit conversion:

The following declarations trigger compiler errors because they require a narrowing conversion:

    int i{ 1.2 };           // error 

    double d = 47 / 13; 
    float f1{ d };          // error 
    float f2{47/13};        // OK

To fix the error, an explicit conversion must be done:

    int i{ static_cast<int>(1.2) }; 

    double d = 47 / 13; 
    float f1{ static_cast<float>(d) };

The following sample shows several examples of direct-list-initialization and copy-list-initialization. In C++11, the deduced type of all these expressions is std::initializer_list<int>.

auto a = {42};   // std::initializer_list<int>
auto b {42};     // std::initializer_list<int>
auto c = {4, 2}; // std::initializer_list<int>
auto d {4, 2};   // std::initializer_list<int>

C++17 has changed the rules for list initialization, differentiating between the direct- and copy-list-initialization. The new rules for type deduction are as follows:

• for copy list initialization auto deduction will deduce a std::initializer_list<T> if all elements in the list have the same type, or be ill-formed.
• for direct list initialization auto deduction will deduce a T if the list has a single element, or be ill-formed if there is more than one element.

Base on the new rules, the previous examples would change as follows: a and c are deduced as std::initializer_list<int>; b is deduced as an int; d, which uses direct initialization and has more than one value in the brace-init-list, triggers a compiler error.

auto a = {42};   // std::initializer_list<int>
auto b {42};     // int
auto c = {4, 2}; // std::initializer_list<int>
auto d {4, 2};   // error, too many

• Using auto whenever possible
• Understanding the various forms of non-static member initialization

Constructors are a place where non-static class member initialization is done. Many developers prefer assignments in the constructor body. Aside from the several exceptional cases when that is actually necessary, initialization of non-static members should be done in the constructor's initializer list or, as of C++11, using default member initialization when they are declared in the class. Prior to C++11, constants and non-constant non-static data members of a class had to be initialized in the constructor. Initialization on declaration in a class was only possible for static constants. As we will see further, this limitation was removed in C++11 that allows initialization of non-statics in the class declaration. This initialization is called default member initialization and is explained in the next sections.

This recipe will explore the ways the non-static member initialization should be done.

To initialize non-static members of a class you should:

• Use default member initialization for providing default values for members of classes with multiple constructors that would use a common initializer for those members (see [3] and [4] in the following code).
• Use default member initialization for constants, both static and non-static (see [1] and [2] in the following code).
• Use the constructor initializer list to initialize members that don't have default values, but depend on constructor parameters (see [5] and [6] in the following code).
• Use assignment in constructors when the other options are not possible (examples include initializing data members with pointer this, checking constructor parameter values, and throwing exceptions prior to initializing members with those values or self-references of two non-static data members).

The following example shows these forms of initialization:

    struct Control 
    { 
      const int DefaultHeigh = 14;                  // [1] 
      const int DefaultWidth = 80;                  // [2] 

      TextVAligment valign = TextVAligment::Middle; // [3] 
      TextHAligment halign = TextHAligment::Left;   // [4] 

      std::string text; 

      Control(std::string const & t) : text(t)       // [5] 
      {} 

      Control(std::string const & t, 
        TextVerticalAligment const va, 
        TextHorizontalAligment const ha):  
      text(t), valign(va), halign(ha)                 // [6] 
      {} 
    };

Non-static data members are supposed to be initialized in the constructor's initializer list as shown in the following example:

    struct Point 
    { 
      double X, Y; 
      Point(double const x = 0.0, double const y = 0.0) : X(x), Y(y)  {} 
    };

Many developers, however, do not use the initializer list, but prefer assignments in the constructor's body, or even mix assignments and the initializer list. That could be for several reasons--for larger classes with many members, the constructor assignments may look easier to read than long initializer lists, perhaps split on many lines, or it could be because they are familiar with other programming languages that don't have an initializer list or because, unfortunately, for various reasons they don't even know about it.

Using assignments in the constructor is not efficient, as this can create temporary objects that are later discarded. If not initialized in the initializer list, non-static members are initialized via their default constructor and then, when assigned a value in the constructor's body, the assignment operator is invoked. This can lead to inefficient work if the default constructor allocates a resource (such as memory or a file) and that has to be deallocated and reallocated in the assignment operator:

    struct foo 
    { 
      foo()  
      { std::cout << "default constructor" << std::endl; } 
      foo(std::string const & text)  
      { std::cout << "constructor '" << text << "'" << std::endl; } 
      foo(foo const & other)
      { std::cout << "copy constructor" << std::endl; } 
      foo(foo&& other)  
      { std::cout << "move constructor" << std::endl; }; 
      foo& operator=(foo const & other)  
      { std::cout << "assignment" << std::endl; return *this; } 
      foo& operator=(foo&& other)  
      { std::cout << "move assignment" << std::endl; return *this;} 
      ~foo()  
      { std::cout << "destructor" << std::endl; } 
    }; 

    struct bar 
    { 
      foo f; 

      bar(foo const & value) 
      { 
        f = value; 
      } 
    }; 

    foo f; 
    bar b(f);

The preceding code produces the following output showing how data member f is first default initialized and then assigned a new value:

default constructor 
default constructor 
assignment 
destructor 
destructor

Changing the initialization from the assignment in the constructor body to the initializer list replaces the calls to the default constructor plus assignment operator with a call to the copy constructor:

    bar(foo const & value) : f(value) { }

Adding the preceding line of code produces the following output:

default constructor 
copy constructor 
destructor 
destructor

For those reasons, at least for other types than the built-in types (such as bool, char, int, float, double or pointers), you should prefer the constructor initializer list. However, to be consistent with your initialization style, you should always prefer the constructor initializer list when that is possible. There are several situations when using the initializer list is not possible; these include the following cases (but the list could be expanded with other cases):

• If a member has to be initialized with a pointer or reference to the object that contains it, using the this pointer in the initialization list may trigger a warning with some compilers that it is used before the object is constructed.
• If you have two data members that must contain references to each other.
• If you want to test an input parameter and throw an exception before initializing a non-static data member with the value of the parameter.

Starting with C++11, non-static data members can be initialized when declared in the class. This is called default member initialization because it is supposed to represent initialization with default values. Default member initialization is intended for constants and for members that are not initialized based on constructor parameters (in other words members whose value does not depend on the way the object is constructed):

    enum class TextFlow { LeftToRight, RightToLeft }; 

    struct Control 
    { 
      const int DefaultHeight = 20; 
      const int DefaultWidth = 100; 

      TextFlow textFlow = TextFlow::LeftToRight; 
      std::string text; 

      Control(std::string t) : text(t) 
      {} 
    };

In the preceding example, DefaultHeight and DefaultWidth are both constants; therefore, the values do not depend on the way the object is constructed, so they are initialized when declared. The textFlow object is a non-constant non-static data member whose value also does not depend on the way the object is initialized (it could be changed via another member function), therefore, it is also initialized using default member initialization when it is declared. text, on the other hand, is also a non-constant non-static data member, but its initial value depends on the way the object is constructed and therefore it is initialized in the constructor's initializer list using a value passed as an argument to the constructor.

If a data member is initialized both with the default member initialization and constructor initializer list, the latter takes precedence and the default value is discarded. To exemplify this, let's again consider the foo class earlier and the following bar class that uses it:

    struct bar 
    { 
      foo f{"default value"}; 

      bar() : f{"constructor initializer"} 
      { 
      } 
    }; 

    bar b;

The output differs, in this case, as follows, because the value from the default initializer list is discarded, and the object is not initialized twice:

constructor
constructor initializer
destructor