[RESOLVED] How come cout won't accept an object if it's been returned from a function?

[tleave2000]

I'm making a fairly simple matrix class and it's mostly working so far but I've found a curiosity when sending it to cout.

It seems like if I create a matrix4 object and then send it to cout, that works fine. But if I create the matrix4 as I send it to cout, then I get an error.

If I uncomment the penultimate line of the code below, I get a huge error message. But I would have thought that it was equivalent to the line above it, which works fine.

The get_transpose() function, the overloaded << operator and the start of the error message look like this:

matrix4 get_transpose() const;

std::ostream& operator << ( std::ostream& stream, matrix4& m );  

matrix4_test.cpp:26: error: no match for 'operator<<' in 'std::cout << matrix4::get_transpose() const()'

Any ideas why it's going wrong or how I can fix it so that I can send matrix4s directly to cout the same as normal types? I've attached the relevant files in case they help.

Here's the problem code, the penultimate line in particular:

#include <iostream>
#include "matrix4.h"

matrix4 get_transpose()	const;

int main()
{
    float a[4][4] = { 
				0,	1,	2,	3,
				4,	5,	6,	7,
				8,	9,	10,	11,
				12,	13,	14,	15
			    };

    matrix4 mat2(a);
	
    matrix4 transpose = mat2.get_transpose();
	
    std::cout << mat2 << "\n\n";
    std::cout << transpose << "\n\n";
    //std::cout << mat2.get_transpose() << "\n\n";
	
    return(0);
}

I'm using MinGW btw. Thanks for reading.

[ZuK]

usually the operator << takes the opject by const reference
e.g.

std::ostream& operator << ( std::ostream& stream, const matrix4& m );

guess your error has to do that you call operator << on a temporary object. If that is true then changing the operator << should help.
Kurt

[Lindley]

Indeed, a temporary can be bound to a const reference but not to a non-const reference. Hence the trouble.

[tleave2000]

Thanks a lot that's sorted it out. Couldn't have hoped for clearer answers.