将map <> :: iterator传递为map <> :: const_iterator＆

Question

I have a problem passing a map<...>::iterator object to a function as a const_iterator & on GCC:

class MyClass {


};

bool MyClass::_GetInstList(map<string,InstList>::const_iterator & it, const string & sMod)
{
    cout<<"Matched\n";
    it = tInstancesData.find(sMod);
    if( it == tInstancesData.end() ) {
        cout<<"\""<<sMod<<"\" is NOT a module\n";
        return false;
    }
    return true;
}

    bool SomeFunction()
    {

            map<string,InstList>::iterator  it;
            if( ! _GetInstList(it, SomeString) ) return false;
            it->second.Add(...);  //  Modifying element pointed by "it"

    }

My probelm is that on Visual Studio 2010 the code above works perfectly fine, but on GCC 4.1.2 I get an error saying there is no matching function to the function call, for _GetInstList(it, SomeString). The issue seems to be converting iterator to const_iterator &.

I have to take it by reference because "it" gets changed inside _GetInstList() and the caller function checks it. (The "it" pointer is changed not a pointed element).

Also, the "it" in SomeFunction() cannot be const because it changes an element.

How can I resolve this?

EDIT: For those who suggest that the problem is the conversion from iterator to const_iterator: The code compiles fine if the function prototype is changed to take const_iterator NOT as a reference, the problem is the const &.

Answer 1

Change your argument type to const map<string,InstList>::const_iterator& or just a map<string,InstList>::const_iterator .

Here's an example demonstrating your problem (and the solution) with simple types:

void func1(double& x){}
void func2(const double& x){}

int main()
{
    int x;
    func1(x); // error: 'func1' : cannot convert parameter 1 from 'int' to 'double &'
    func2(x); // succeeds
}

Answer 2

I think maybe this method should be redesigned anyway. Passing iterators around is messy and confusing for others to read. How hard would it be to do that?