C++ templates problem

I am new to templates in c++. i was trying some small programs.

    CPP [80]> cat 000001.cpp 000001.hpp
#include <iostream>
#include <string>
#include "000001.hpp"

int main()
{
    int i = 42;
    std::cout << "max(7,i):   " << ::max(7,i) << std::endl;

    double f1 = 3.4;
    double f2 = -6.7;
    std::cout << "max(f1,f2): " << ::max(f1,f2) << std::endl;

    std::string s1 = "mathematics";
    std::string s2 = "math";
    std::cout << "max(s1,s2): " << ::max(s1,s2) << std::endl;
}

template <typename T>
inline T const& max (T const& a, T const& b)
{
        return  a < b ? b : a;
}

when i compile this program:

i get an error below:

    CPP [78]> /opt/aCC/bin/aCC -AA 000001.cpp
Error (future) 229: "/opt/aCC/include_std/string.cc", line 164 # "Ambiguous overloaded function call; a
    functi开发者_运维百科on match was not found that was strictly best for ALL arguments. Two functions that matched
    best for some arguments (but not all) were "const unsigned long &max<unsigned long>(const unsigned
    long &,const unsigned long &)" ["000001.hpp", line 2] and "const unsigned long &std::max<unsigned
    long>(const unsigned long &,const unsigned long &)" ["/opt/aCC/include_std/algorithm", line 1762]."
    Choosing "const unsigned long &max<unsigned long>(const unsigned long &,const unsigned long &)"
    ["000001.hpp", line 2] for resolving ambiguity.
            _C_data = _C_getRep (max (_RW::__rw_new_capacity (0, this),
                                 ^^^
Warning:        1 future errors were detected and ignored. Add a '+p' option to detect and fix them before they become fatal errors in a future release. Behavior of this ill-formed program is not guaranteed to match that of a well-formed program

Could nybody please tell me what exactly the error is?

You are probably including <iostream.h> instead of <iostream> somewhere. The former hasn't existed for some time now, but for compatibility reasons, you compiler still accepts the include and replaces it with

#include <iostream>
using namespace std;

This causes std::max to be brought to the global namespace, thus resulting in an ambiguity. Replace <iostream.h> with <iostream> or rename your max function and the problem should disappear.

Edit: You've apparently fixed the include, but I bet you still have using namespace std; somewhere. You need to get rid of that. In fact you should never use using namespace in the global scope.

Edit: You might also have using std::max somewhere. You need to get rid of it too.

The code you've posted compiles just fine, there must be something else that is wrong inside "000001.hpp". Can you post the contents of that file too?

Edit: If you do as avakar says but the problem persists, that must be due to some problem with your compiler. There are two obvious workarounds I can think of: rename your max function to something else, or put it in a namespace:

namespace Foo
{
    template <typename T>
    inline T const& max (T const& a, T const& b)
    {
        return  a < b ? b : a;
    }
}

int main()
{
    int i = 42;
    std::cout << "max(7,i):   " << Foo::max(7,i) << std::endl;

    double f1 = 3.4;
    double f2 = -6.7;
    std::cout << "max(f1,f2): " << Foo::max(f1,f2) << std::endl;

    std::string s1 = "mathematics";
    std::string s2 = "math";
    std::cout << "max(s1,s2): " << Foo::max(s1,s2) << std::endl;
}

I don't know which compiler you are using but the second error tells you that the two following functions are clashing :

::max
std::max

It seems really weird, you may have a using namespace std; somewhere or worse, that one of your include use iostream.h as noted in the first error. Could you give more information about your compiler/toolchain and the content of your .hpp file?

It says that two definitions for

max<unsigned long>

were found. One definition is in 000001.hpp and the other is in /opt/aCC/include_std/algorithm. The compiler chose the one in 000001.hpp for now, so no error is present now. But it says that these two definitions may cause errors in the future.

I don't know if this causes the problem, but anyhow; you shouldnt use the name max for your global (or local) function as it is a part of STL.