Using std::map where V has no usable default constructor

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You can’t make the compiler differentiate between the two uses of operator[], because they are the same thing. Operator[] returns a reference, so the assignment version is just assigning to that reference.

Personally, I never use operator[] for maps for anything but quick and dirty demo code. Use insert() and find() instead. Note that the make_pair() function makes insert easier to use:

m.insert( make_pair( k, v ) );

In C++11, you can also do

m.emplace( k, v );
m.emplace( piecewise_construct, make_tuple(k), make_tuple(the_constructor_arg_of_v) );

even if the copy/move constructor is not supplied.

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