G ++ std :: function问题

Question

I have the following code:

#include <functional>

std::function<int,int> p;

int main()
{

return 0;
}

I am using MinGW g++ 4.8.1 which fails with

C:\main.cpp|4|error: wrong number of template arguments (2, should be 1)|

c:\mingw\lib\gcc\mingw32\4.8.1\include\c++\functional|1866|error: provided for 'template<class _Signature> class std::function'|

C:\main.cpp|4|error: invalid type in declaration before ';' token|

is this a G++ bug or am I using std::function incorrectly

Answer 1

std::function<int(int)> for function takes int and return int. eg

int foo(int);

std::function<void(int,int)> for function takes two ints and no return value. eg

void foo(int, int);

Answer 2

std::function takes one template argument - the type of the callable object that it wraps around. So, if you want to construct an std::function which returns type Ret and takes arguments of types Arg1, Arg2,..., ArgN , you would write std::function<Ret(Arg1, Arg2,..., ArgN)> .

(Note that the ellipses weren't meant to indicate parameter pack expansion - they are just being used in the regular mathematical sense.)

Answer 3

As the compiler says, std::function takes one template argument.

Use the syntax returntype(argtype, ...)

int foo(int a, int b) { return a+b; }
std::function<int(int,int)> p = foo;

int bar(int a) { return ++a; }
std::function<int(int)> q = bar;

void boo() { return; }
std::function<void()> r = boo;