Fold expressions
A fold expression is an expression involving a parameter pack that folds (or reduces) the elements of the parameter pack over a binary operator. To understand how this works, we will look at several examples. Earlier in this chapter, we implemented a variable function template called sum that returned the sum of all its supplied arguments. For convenience, we will show it again here:
template <typename T>
T sum(T a)
{
return a;
}
template <typename T, typename... Args>
T sum(T a, Args... args)
{
return a + sum(args...);
}
With fold expressions, this implementation that requires two overloads can be reduced to the following form:
template <typename... T>
int sum(T... args)
{
return (... + args);
}
There is no need for overloaded functions anymore. The expression (... + args) represents the fold expression, which upon evaluation becomes ((((arg0 + arg1) + arg2) + … ) + argN...
