Re: Completely misleading declaration

James Kanze <>
Wed, 19 Mar 2008 04:50:42 -0700 (PDT)
Juha Nieminen wrote:

Assume we have a piece of code like this:

#include <map>

struct A { ... };
struct Comp { bool operator()(const A&, const A&) const { ... } };

int main()
    std::map<A, int, Comp> theMap(Comp());

    A a;
    theMap[a] = 5;

  All is well, right? No. gcc gives this error:

error: no match for 'operator[]' in 'theMap[a]'

  Inexperienced C++ programmers will be completely confused by this
error message.

Even experienced C++ programmers might be confused with it.
Given the frequency of this error, a compiler can be more
explicit about it. (I seem to recall g++ also outputting
information about the types it was working with.)

Experienced C++ programmers may see what is the problem: What
looks like a map instantiation called 'theMap' actually isn't.
It actually is a function declaration. This even though we are
seemingly creating a (nameless) instance of Comp which we give
to this map as parameter. How can a function be declared with
a parameter which is an instance of a struct instead of being
a parameter type?

Because for historical reasons, C++ declaration syntax is
broken, and there's no easy way to fix it without breaking
existing code.

It seems that we have a doubly-confusing declaration here. In
fact, in this case it doesn't seem to be an instantiation of
Comp at all. Instead, at least according to gcc, it declares
a function pointer type (!)

  So, in all its glory, the line:

    std::map<A, int, Comp> theMap(Comp());

declares a function named 'theMap' which returns an instance
of std::map<A, int, Comp> and which takes one parameter: A
pointer to a function taking no parameters and which returns
an instance of Comp.

Could this become any more confusing?

A small change in that line completely changes its semantic

    std::map<A, int, Comp> theMap((Comp()));

Now it does what it looks like it should do: It creates a map
instance called 'theMap' and gives its constructor an instance
of Comp.

Anyways, my actual question is the following:

Is gcc behaving correctly here? Can you really declare a
function pointer type as "Comp()" instead of the more common
"Comp (*)()"?

Yep. You've just run into a case of the most embarassing parse.
I don't think anyone likes the current situation, but to date,
no one has been able to propose an acceptable alternative.

James Kanze (GABI Software)
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