std::make_pair

[Joeman]

this compiles

#include <utility>
#include <Windows.h>

int main()
{
    RECT rc = {0};
    std::pair <LONG,LONG> ControlSize;
    ControlSize = std::make_pair(rc.right - rc.left,rc.bottom - rc.top);
    return 0;
}

this does not

#include <utility>
#include <Windows.h>

int main()
{
    RECT rc = {0};
    const std::pair <LONG,LONG> ControlSize;
    ControlSize = std::make_pair(rc.right - rc.left,rc.bottom - rc.top);
    return 0;
}

error C2678: binary '=' : no operator found which takes a left-hand operand of type 'const std::pair<_Ty1,_Ty2>' (or there is no acceptable conversion)

Notice the const. Once the ControlSize has been initialized, there is no changing it without breaking const, but you can initialize it with make_pair like

#include <utility>
#include <Windows.h>

int main()
{
    RECT rc = {0};
    const std::pair <LONG,LONG> ControlSize = std::make_pair(rc.right - rc.left,rc.bottom - rc.top);
    return 0;
}