On the up side: my 1st linker error in the whole tutorial. W00!

Oh, and great tutorial Alex (if you still read these). I even stopped reading the book I was reading on C++ about halfway through and totally moved onto this and even started doing some graphics work at the same time because the information presented here was so clear and concise albeit littered with “it’s” when they should have been “its” :P. I will go back and finish the book at some point just in case I missed anything, but in the mean time onward I go.

A note for users using older compilers

Some older compilers (eg. Visual Studio 6) have a bug where the definition of template class functions must be put in the same file as the template class is defined in. Thus, if the template class were defined in X.h, the function definitions would have to also go in X.h (not X.cpp). This issue should be fixed in most/all modern compilers.

I am afraid this is still the case with Visual Studio 2008; if I just completely define the function in the .h file, it works as expected.

// .h file def
template <typename T>
int GetArrayLength(T iArray[]);

// .cpp file function
template <typename T> // this is the template parameter declaration
int GetArrayLength(T iArray[])
{
	return sizeof(iArray) / sizeof(iArray[0]);
}

If i then use the GetArrayLength() function from my main.cpp, I get the following build errors:

Error	1	error LNK2019: unresolved external symbol "int __cdecl GetArrayLength<int>(int * const)" (??$GetArrayLength@H@@YAHQAH@Z) referenced in function _wmain	CPPTest.obj	CPPTest
Error	2	fatal error LNK1120: 1 unresolved externals	CPPTest.exe	1	CPPTest

If I just define the function twice (once in my helper.cpp and once in my main.cpp), everything works as expected. Any idea what’s wrong here?

For example:

using namespace std;

template  // this is the template parameter declaration
Type max(Type tX, Type tY)
{
    return (tX > tY) ? tX : tY;
}

int main()
{
	int nValue = max(3, 7); // returns 7
	double dValue = max(6.34, 18.523); // returns 18.523
	char chValue = max('a', '6'); // returns 'a'
	cout << nValue << " " << dValue << " " << chValue << endl;
	return 0;
}

This creates an error while this doesn’t:

template  // this is the template parameter declaration
Type max(Type tX, Type tY)
{
    return (tX > tY) ? tX : tY;
}

int main()
{
	int nValue = max(3, 7); // returns 7
	double dValue = max(6.34, 18.523); // returns 18.523
	char chValue = max('a', '6'); // returns 'a'
        using namespace std;
	cout << nValue << " " << dValue << " " << chValue << endl;
	return 0;
}

[ This is because the local version of max() has a namespace collision with std::max(). I made a note of it at the bottom of the article. Thanks for the heads up. -Alex ]

int max(double dX, double dY)
{
    return (dX > dY) ? dX : dY;
}

the return should be a double not an int, so:

double max(double dX, double dY)
{
    return (dX > dY) ? dX : dY;
}

is better

[ Yes, thank you. It's fixed now. -Alex ]