vector<string> warnings

**R.Stiltskin** · 10-26-2005

This program serves no purpose except to start learning how to use some STL classes.

It compiles & runs as expected, but with some ugly & incomprehensible warnings (actually 5 copies of the same warning). What do they mean, and why 5 of them?

Code:

#include <iostream>
#include <vector>
#include <string>

using namespace std;

int main(){
	int i;
	vector<string> list;
	string myString("seven");
    list.push_back("six");
    list.push_back("one");
    list.push_back("two");
    list.push_back("three");
    list.push_back("four");
    list.push_back("five");
    for(i=0;i<6;i++)
        cout << (list.at(i)).at(0) << endl;
	cout << myString << endl;
	vector<string> list2 (6, myString);
	for(i=0;i<list2.size();i++)
		cout << list2.at(i) << endl;

    return 0;
}

--------------------Configuration: vector2 - Win32 Debug--------------------
Compiling...
vector2.cpp
E:\cs71010\scrap\vector2\vector2.cpp(25) : warning C4786: 'std::reverse_iterator<std::basic_string<char,std: :char_traits<char>,std::allocator<char> > const *,std::basic_string<char,std::char_traits<char>,st d::allocator<char> >,std::basic_string<char
,std::char_traits<char>,std::allocator<char> > const &,std::basic_string<char,std::char_traits<char>,st d::allocator<char> > const *,int>' : identifier was truncated to '255' characters in the debug information
E:\cs71010\scrap\vector2\vector2.cpp(25) : warning C4786: 'std::reverse_iterator<std::basic_string<char,std: :char_traits<char>,std::allocator<char> > *,std::basic_string<char,std::char_traits<char>,st d::allocator<char> >,std::basic_string<char,std::
char_traits<char>,std::allocator<char> > &,std::basic_string<char,std::char_traits<char>,st d::allocator<char> > *,int>' : identifier was truncated to '255' characters in the debug information
c:\program files\microsoft visual studio\vc98\include\vector(39) : warning C4786: 'std::vector<std::basic_string<char,std::char_trai ts<char>,std::allocator<char> >,std::allocator<std::basic_string<char,std::char_ traits<char>,std::allocator<char> > >
>::vector<std::basic_string<char,std::char_traits< char>,std::allocator<char> >,std::allocator<std::basic_string<char,std::char_ traits<char>,std::allocator<char> > > >' : identifier was truncated to '255' characters in the debug information
c:\program files\microsoft visual studio\vc98\include\vector(47) : warning C4786: 'std::vector<std::basic_string<char,std::char_trai ts<char>,std::allocator<char> >,std::allocator<std::basic_string<char,std::char_ traits<char>,std::allocator<char> > >
>::vector<std::basic_string<char,std::char_traits< char>,std::allocator<char> >,std::allocator<std::basic_string<char,std::char_ traits<char>,std::allocator<char> > > >' : identifier was truncated to '255' characters in the debug information
c:\program files\microsoft visual studio\vc98\include\vector(60) : warning C4786: 'std::vector<std::basic_string<char,std::char_trai ts<char>,std::allocator<char> >,std::allocator<std::basic_string<char,std::char_ traits<char>,std::allocator<char> > >
>::~vector<std::basic_string<char,std::char_traits <char>,std::allocator<char> >,std::allocator<std::basic_string<char,std::char_ traits<char>,std::allocator<char> > > >' : identifier was truncated to '255' characters in the debug information

vector2.obj - 0 error(s), 5 warning(s)

**Dave_Sinkula** · 10-26-2005

http://msdn.microsoft.com/library/de...err46xx_49.asp

Perhaps search the board for C4786 as well.

**R.Stiltskin** · 10-26-2005

Oops. Sorry about the spam.

So Microsoft's compiler is complaining about the length of the name of Microsoft's template class?

**R.Stiltskin** · 10-26-2005

Anyway, what is all that stuff?

How did vector<string> grow to

std::reverse_iterator<std::basic_string<char,std:: char_traits<char>,std::allocator<char> > const *,std::basic_string<char,std::char_traits<char>,st d::allocator<char> >,std::basic_string<char
,std::char_traits<char>,std::allocator<char> > const &,std::basic_string<char,std::char_traits<char>,st d::allocator<char> > const *,int>'

Actually the 5 identifiers are not all the same. I think the last 3 are the same; the first 2 are different.

**CornedBee** · 10-27-2005

std::string is actually a typedef for a concrete instantiation of the template basic_string.

The standard defines basic_string as

Code:

template < typename CharacterType, typename CharacterTraits, typename Allocator >

CharacterType is the data type that makes up a single character. Usually this will be char or wchar_t, but in theory you can use anything.
CharacterTraits defines some basic properties of the character type, like how they are to be compared for equality. This parameter defaults to std::char_traits<CharacterType>.
Allocator is responsible for memory allocation. This defaults to std::allocator<CharacterType>.

std::string is defined as such:

Code:

typedef basic_string<char> string;

... using the defaults for the other two parameters.

Supplying the full template substitution, complete with namespace prefixes, this is the resulting type:

Code:

std::basic_string< char, std::char_traits< char >, std::allocator< char > >

That's not too bad. Now, vector is a template, too. vector<string> expands to this:

Code:

std::vector< std::string, std::allocator< std::string > >

and substituting std::string:

Code:

std::vector<
  std::basic_string<
    char,
    std::char_traits<
      char
    >,
    std::allocator<
      char
    >
  >,
  std::allocator<
    std::basic_string<
      char,
      std::char_traits<
        char
      >,
      std::allocator<
        char
      >
    >
  >
>

That alone is enough to break the limit on identifier length that VC++6 imposed.

But there's also the implementation details of std::vector, such as the instantiation of

Code:

std::reverse_iterator<
  typename Iterator,
  typename ValueType = std::iterator_traits<Iterator>::value_type,
  typename Reference = std::iterator_traits<Iterator>::reference,
  typename Pointer = std::iterator_traits<Iterator>::pointer,
  typename Difference = std::iterator_traits<Iterator>::difference_type
>

with Iterator = std::string const * (expanded, of course).

**R.Stiltskin** · 10-27-2005

Thanks, CornedBee, but that's pretty baffling. Can you suggest any websites where I can find explanations and examples of how those template arguments other than <class Type> are actually used and what they do? (Hopefully something clearer than I can find in the MSDN Library.)

**Prelude** · 10-27-2005

You'd be better off ignoring them for now. It's entirely possible that you'll never need to consider them in your own code.

**CornedBee** · 10-28-2005

If you do want to know, I'd recommend a book, like "The Standard Template Library" by Nicolai Josutti (name taken from memory, might be wrong).

**R.Stiltskin** · 10-28-2005

Thanks, I will look at that.

BTW, for anyone who's interested, it's Nicolai M. Josuttis, and the isbn is 0-201-37926-0.

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