Differences between compilers

[Stewbond]

This function will compile and operate perfectly with VS2010 and VS2008, but give an error with GNU GCC/g++ compiler:

#include <vector>
#include <iterator>
#include <algorithm>

// Returns the median of all elements in the range [first, last)
template <class Iter>
typename std::iterator_traits<Iter>::value_type
    Median(Iter first, Iter last)
{
    std::vector<std::iterator_traits<Iter>::value_type> arr( first, last );

    std::sort( arr.begin(), arr.end() );

    return arr.at( arr.size()/2 );
}

To make it compile, I had to add typename to line 10 like so:

#include <vector>
#include <iterator>
#include <algorithm>

// Returns the median of all elements in the range [first, last)
template <class Iter>
typename std::iterator_traits<Iter>::value_type
    Median(Iter first, Iter last)
{
    std::vector<typename std::iterator_traits<Iter>::value_type> arr( first, last );

    std::sort( arr.begin(), arr.end() );

    return arr.at( arr.size()/2 );
}

Which one is standard C++? What is the rule on this? How do I know that my code will work on any compiler?

[Thumper]

AFAIK, typename is required before a qualified dependent name that refers to a type EDIT: that's used inside of a template, ofc.
value_type is a dependent name that refers to a type. Therefore, must be prefixed with typename.

The only way to be sure you code will work on every compiler is to either adhere to very strict C++ standards (and even then you might run into problems because some compilers might have more or less support for something than another. C++11, for example, has varying levels of support across compilers), or test compile your code on several different compilers.

[zb0S216C]

The second code snippet is the correct one and it's the standard way.

The reason why you need a "typename" in that position is because "std::iterator::value_type" is a dependent name. Because templates are incomplete types before instantiation, the compiler cannot assume "std::iterator::value_type's" identity until "std::iterator" has been instantiated with some complete type. Because of the aforementioned, "typename" is used to tell the compiler in advance (prior to instantiation) that some nested symbol is in fact a type and not a piece of storage or function.

_Wazzak

[Stewbond]

Ah excellent! Thanks for the info.

VS must have some additional disambiguation so it's good to know what I need to add to make it work on multiple compilers.

As a side note, and of interest, I recently found out that <cstdint> is not included in VS2008 despite being in C++11 and C99. VS2008 is not compliant with either standard. To make my code work there, I had to get rid of the useful uint32_t and replace it with a typedef of my own.

[Thumper]

Ah, yeah. Visual studio didn't have stdint.h until VS2010. Although, you can copy an existing one into a VS2008 install if you feel like going the extra mile.
Visual Studio compilers are known for bending the rules a bit.