C++ Template Weird problem, please help

[usmandastgeer]

Hello,

I am using C++ templates and working on a library where a problem with C++ templates appeared. I simulated that error here with simple program for your help.


#include <iostream>

using namespace std;

class Vector
{
public:
void X()
{
cout<<"X in Vector\n";
}
};


class Matrix
{
public:
void X(int m)
{
cout<<"X in Matrix\n";
}
};


template <typename T>
int gg(T a, bool tt)
{
if(tt)
a.X();
else
a.X(21);
}

int main()
{
Vector v;
Matrix m;
gg(v, true);
gg(m, false);
}


Now using gcc on linux, it compiles that cannot find V.X(int) for Vector instantiation which should not be as it is in else block. Exact errors are as below:

test.cpp: In function ‘int gg(T, bool) [with T = Vector]’:
test.cpp:38: instantiated from here
test.cpp:31: error: no matching function for call to ‘Vector::X(int)’
test.cpp:8: note: candidates are: void Vector::X()
test.cpp: In function ‘int gg(T, bool) [with T = Matrix]’:
test.cpp:39: instantiated from here
test.cpp:29: error: no matching function for call to ‘Matrix::X()’
test.cpp:18: note: candidates are: void Matrix::X(int)



Please help me as I don't why compiler do it. Even I have tried using bool as template parameter but still it was expanding else block also. What should be a viable alternative???

Thanks in advance

[Paul McKenzie]

First, please use code tags when posting code. The code you posted is hard to read and unformatted.

Second,

a.X();

The error is exactly as described. This function does not exist in the Matrix class when you attempted to call gg(m, false).

Templates are compile-time constructs, they are not runtime constructs. That if() statement is executed at runtime, so it isn't going to help you.

One way to solve this problem is to use template specialization, and specialize on bool.

#include <iostream>

using namespace std;

class Vector
{
public:
	void X()
	{
		cout<<"X in Vector\n";
	}
};


class Matrix
{
public:
	void X(int m)
	{
		cout<<"X in Matrix\n";
	}
};

template <typename T, bool isTT>
struct ggStruct
{
	static int gg(T a)
	{
		a.X();
		return 1;
	}     
};

template <typename T>
struct ggStruct<T, false>
{
	static int gg(T a)
	{
		a.X(21);
		return 1;
	}     
};

int main()
{
	Vector v;
	Matrix m;
	ggStruct<Vector, true>::gg(v);
	ggStruct<Matrix, false>::gg(m);
}

Note that we need to use struct/classes to do the specialization, as you cannot specialize template functions.

Regards,

Paul McKenzie

[potatoCode]

First, this is not a template related problem.

if-else is conditional statements for which the compiler must account for EVERY probabilities.
The error message is saying that the compiler is clueless if you had made a call to gg(v, false) which requires a Vector class member function that takes integral type (but there is none)

A quick fix would be to give a default argument value for both

void X(int val = 0) { ... } // Vecotr
...
void X(int val = 0) { ... } // matrix

However, I doubt that the actual problem in your application can be soloved in this way. A better approach is to overload void X(), or specialize it if Vector is a class template, but don't mix-and-match the two for the free function. If none of these is a viable option, it's time to rethink the whole design.