Re: compilation error related to template parameter

"Alf P. Steinbach" <>
Tue, 14 Aug 2007 08:55:23 +0200
*, India:

Consider the following program:

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

using namespace std;

template<class T> class Vec : public vector<T>
Vec() : vector<T>() { }

Vec(int s) : vector<T>(s) { }

T& operator[] (int i) { return at(i); }

int main()
      Vec<string> s(3);

       return 0;

Suppose this program is named as y.cpp

When this program is compiled under Redhat Enterprise Linux
Workstation, as
g++ -std=c++98 -pedantic -Wall -Wextra y.cpp

I am getting the following compilation error:

y.cpp: In member function `T& Vec<T>::operator[](int)':
y.cpp:14: error: there are no arguments to `at' that depend on a
template parameter, so a declaration of `at' must be available

However under VC++ Express Edition 2005, it compiles well without any
warning or error.

Kindly explain what is wrong with the above program and help me in
fixing the compilation error with g++ under Linux

As an alternative to Red Floyd's suggestion (else-thread) of

   this->at( i )

you can add in the class

   using vector<T>::at;

For the compiler needs to be informed -- somehow -- that "at" is meant
to be assumed to be a member function of vector<T>. Because whether it
is or not depends on T, which is unknown during the first pass through
the template definition. For example, "at" could be a global function.

I always find the explanation I gave above to be lacking, but it is the
one usually offered. It's a bit lacking because before two-phase
template instantiation was standardized, compilers managed code such as
above very well thank you, as evidently Visual C++ still does, without
needing to be informed about anything. A more Real(TM) reason why it
matters could be that "export" needs a context-independent template
definition (that could also help explain why "export" is so unpopular,
only officially supported by Comeau).

A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
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