insert vs emplace vs operator[] in c++ map

Question

I\'m using maps for the first time and I realized that there are many ways to insert an element. You can use emplace(), operator[] or insert(

Answer 1

Apart from the optimisation opportunities and the simpler syntax, an important distinction between insertion and emplacement is that the latter allows explicit conversions. (This is across the entire standard library, not just for maps.)

Here's an example to demonstrate:

#include 

struct foo
{
    explicit foo(int);
};

int main()
{
    std::vector v;

    v.emplace(v.end(), 10);      // Works
    //v.insert(v.end(), 10);     // Error, not explicit
    v.insert(v.end(), foo(10));  // Also works
}

This is admittedly a very specific detail, but when you're dealing with chains of user-defined conversions, it's worth keeping this in mind.