std::make_pair

[NLscotty]

Hello,

I'm using std::make_pair, to fill a std:air. I only get some error and I can't find out why.

std::pair <LONG,LONG> ControlSize;
ControlSize = std::make_pair(rc.right - rc.left,rc.bottom - rc.top);

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

Grz

[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;
}

[NLscotty]

I never use const infront of it. I use it as the first example and it doesn NOT work. That's what is so weird, or there is a reason I don't know yet.

[Paul McKenzie]

I never use const infront of it. I use it as the first example and it doesn NOT work. That's what is so weird, or there is a reason I don't know yet.

If you get a compiler error, you should post a minimal, but complete code example. Your example is incomplete -- it's just two lines copied from some function.

You should also post the compiler your using, maker and version.

Here is a complete example that compiles successfully:

#include <map>

int main()
{
    std::pair <long, long> ControlSize;
    ControlSize = std::make_pair(0, 0);
}

This was compiled using the online Comeau C++ compiler. There are no errors in this code.

Regards,

Paul McKenzie

[NLscotty]

Oki, ill post some more code

class WinTraits 
{
public:
	WinTraits();
//,,,,,,,,,,,,,,,
//,,,,,,,,,,,,,,,

private:

std::pair <LONG,LONG> ControlSize;
//...............
//................
};

WinTraits::WinTraits()
	:	//.................
		ControlSize(0,0)
{
//..........
}

std::pair<LONG,LONG> WinTraits::GetSize() const
{
	if(!hwnd)
		return ControlSize;
	
	RECT rc = {0};
	::GetWindowRect(hwnd, &rc);
	ControlSize =  std::make_pair<LONG,LONG>(rc.right - rc.left,rc.bottom - rc.top);
        return ControlSize
}

That's all the code with 'ControlSize'. Hope this helps.

[SkyNetTo]

std::pair<LONG,LONG> WinTraits::GetSize() const

you have choosen not to modify any class member by placing that const in red, but you have just modified one.

[NLscotty]

std::pair<LONG,LONG> WinTraits::GetSize() const

you have choosen not to modify any class member by placing that const in red, but you have just modified one.

Omg, so stupid. Thanks a lot