Question

I ran into some trouble trying to implement a smart equality test macro-type template function in Visual C++ 2010 that had to do with a bug in VS in regard to default arguments of template functions. I fixed it by wrapping the value of the parameter in an extra function, but now I found that I can’t use the function twice in one line!

Header file:

// example.h
#pragma once

#include <limits>

namespace myspace
{

// Need to define this separately to avoid a Visual Studio bug
template<typename T> T epsilon() { return std::numeric_limits<T>::epsilon(); }

// A generic equality test
template<typename T> inline bool smartEqual(
    const T &v1, 
    const T &v2, 
    const T &eps = epsilon<T>())
{
    return (v1 == v2);
}

// Template specialization for floating-point numbers
template<> bool smartEqual<float>(
    const float &v1, 
    const float &v2, 
    const float &eps);

} // namespace myspace

Source file:

// example.cpp
#include "example.h"

using namespace std;
using namespace myspace;

// equal-macro specialization for floats using epsilon
template<> bool myspace::smartEqual<float>(
    const float &v1, 
    const float &v2, 
    const float &eps)
{
    return (fabs(v1 - v2) < eps);
}

int _tmain(int argc, _TCHAR* argv[])
{
    float a,b;
    bool x = smartEqual(a,b); // works ok
    bool x = smartEqual(a,b) && smartEqual(b,a); // error
    return 0;
}

The error is reported as follows:

—— Build started: Project: test, Configuration: Debug Win32 ——
test.cpp
c:\users\ninja\documents\visual studio 2010\projects\test\test\test.cpp(24): error C2440: ‘default argument’ : cannot convert from ‘const float *’ to ‘const float &’
Reason: cannot convert from ‘const float *’ to ‘const float’
There is no context in which this conversion is possible

The offending line is the one where I try to call smartEqual() twice using the logical AND.

I don’t understand why this happens. Changing “eps” from a reference type to a straightforward value type fixes it, but I wish I knew what was going on.

Thanks!

Answer 1

I think you’ve now hit this VS10 bug.

Your code compiles OK on VS11 Beta.

You could possibly avoid the default value (which seems to be a major issue for VS10) by changing smartEqual to:

template<typename T> inline bool smartEqual(
    const T &v1, 
    const T &v2)
{
    return (v1 == v2);
}

and simply specialising for float (and double) like this:

template<> bool myspace::smartEqual<float>(
    const float &v1, 
    const float &v2)
{
    return (fabs(v1 - v2) < std::numeric_limits<float>::epsilon());
}

Another option is to change the epsilon parameter to pass by value:

template<typename T> inline bool smartEqual(
    const T &v1, 
    const T &v2, 
    T eps = epsilon<T>())
{
    return (v1 == v2);
}