Understanding C++ Template Error

#inc开发者_高级运维lude <iostream>
#include <cstring>
#include <string>

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

inline char const* max (char const* a, char const* b)
{ 
    return  std::strcmp(a,b) < 0  ?  b : a;
}

template <typename T>
inline T const& max (T const& a, T const& b, T const& c)
{
    return max (max(a,b), c);  // error
}

int main ()
{
    ::max(7, 42, 68);     // OK

    const char* s1 = "frederic";
    const char* s2 = "anica";
    const char* s3 = "lucas";
    ::max(s1, s2, s3);    // ERROR

}

Could anybody please tell me why this is an error?

When you say:

 max (max(a,b), c)

max(char*,char*) returns a pointer BY VALUE. You then return a reference to this value. To make this work, you should make all your max() functions return values rather than references, as I think was suggested in an answer to your previous question, or make the char* overload take and return references to pointers.

You're returning a reference to a temporary. The char* overload of max returns by value, but the 3-arg template returns by reference.

I don't know exactly why you get an error. I only get a warning on my compiler (GCC). I imagine if you posted the error text, though, someone could figure it out.

If you really want to return a reference, then you have to modify the second overload to return a reference too.

char const* const & max (char const* const & a, char const* const & b)

Your example is equivalent to this, maybe you'll see it better this way:

int foo()
{
    return 0;
}

int const & bar()
{
    return foo(); // Reference to what???
}

When compiling, I get:

maxtest.cpp: In function `const T& max(const T&, const T&, const T&) [with T = const char*]':
maxtest.cpp:29:   instantiated from here
maxtest.cpp:19: warning: returning reference to temporary

That would be because you're returning a reference to a temporary (i.e. your object no longer exists by the time the caller of the function can check its value). This is certainly a bug but, since you said error, I suspect it isn't the bug you meant.

Some of the headers in Visual Studio #define min and max. To get around that, put parentheses around min and max:

return (max) ( (max) (a, b), c );

gcc 4.2.1 gives a warning about returning reference to a temporary, so, changing your code to return by value helps.

BTW: I also renamed max to my_max, since max is already defined in <algorithm>

#include <iostream>
#include <cstring>
#include <string>

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

inline char const* my_max (char const* a, char const* b)
{
    return  std::strcmp(a,b) < 0  ?  b : a;
}

template <typename T>
inline T const my_max (T const& a, T const& b, T const& c)
{
    return my_max (my_max(a,b), c);  // error
}

int main ()
{
    std::cout << my_max(7, 42, 68) << "\n"; // OK

    const char* s1 = "frederic";
    const char* s2 = "anica";
    const char* s3 = "lucas";
    std::cout << my_max(s1, s2, s3) << "\n"; // NO ERROR

}

gcc -Wall -Wextra file.cpp -o test gives no warnings or errors, output is:

68
lucas