C++ Primer Plus

Fixing the Problem

The function wrapper lets you rewrite the program so that it uses just one instantiation of use_f() instead of five. Note that the function pointers, function objects, and lambda expressions in Listing 18.7 share a common behavior—each takes one type double argument and each returns a type double value. We can say that each has the same call signature, which is described by the return type followed by a comma-separated list of parameter types enclosed in a pair of parentheses. Thus, these six examples all have double(double) as the call signature.

The function template, declared in the functional header file, specifies an object in terms of a call signature, and it can be used to wrap a function pointer, function object, or ...