Re: Inserting objects into a std::map?

From:

Paul Brettschneider <paul.brettschneider@yahoo.fr>

Newsgroups:

comp.lang.c++

Date:

Sat, 29 Mar 2008 12:46:55 +0100

Message-ID:

<bec53$47ee2c2e$5470058e$14124@news.chello.at>

James Kanze wrote:

On Mar 27, 10:04 pm, Paul Brettschneider
<paul.brettschnei...@yahoo.fr> wrote:

saneman wrote:

[...]

Another thing. When I make:

std::map<Bob, int> m;

And my Bob class does not define '<' operator why does the
compiler not complain in the above declaration of 'm'?

Its first when I insert that the compiler complains about
the missing operator in Bob:

It's the way templates work: functions/methods are only
instantiated when you use them. Since Key::operator<() is not
needed for constructing an empty map, the compiler doesn't
complain.

Or it does (g++, for example). The current standard says it's
undefined behavior, so anything the implementation does is
legal.

I cannot reproduce this on g++:

#include <map>

class A {
        int x;
public:
        A(int x_) : x(x_) { };
};

int main()
{
        std::map<A, int> x; // Compiles fine

        A a(123);
        // x[a] = 456; // <-- This fails
}

compiles on my g++4.2 and g++4.1 versions as long as the last line stays
commented out. I'd find it curious if the std::map construction would
instantiate a method depending on the Compare argument.