Question

When using the double variant of the std::abs() function without the std with g++ 4.6.1, no warning or error is given.

#include <algorithm>
#include <cmath>

double foobar(double a)
{
     return abs(a);
}

This version of g++ seems to be pulling in the double variant of abs() into the global namespace through one of the includes from algorithm. This looks like it is now allowed by the standard (see this question), but not required.

If I compile the above code using a compiler that does not pull the double variant of abs() into the global namespace (such as g++ 4.2), then the following error is reported:

warning: passing 'double' for argument 1 to 'int abs(int)'

How can I force g++ 4.6.1, and other compilers that pull functions into the global namespace, to give a warning so that I can prevent errors when used with other compilers?

Answer 1

The function you are using is actually the integer version of abs, and GCC does an implicit conversion to integer.

This can be verified by a simple test program:

#include <iostream>
#include <cmath>

int main()
{
    double a = -5.4321;
    double b = std::abs(a);
    double c = abs(a);

    std::cout << "a = " << a << ", b = " << b << ", c = " << c << '\n';
}

Output is:

a = -5.4321, b = 5.4321, c = 5

To get a warning about this, use the -Wconversion flag to g++. Actually, the GCC documentation for that option explicitly mentions calling abs when the argument is a double. All warning options can be found here.