Are there any cases where it is incorrect to replace push_back with emplace_back?

Question

Can I break a valid C++03 program by replacing std::vector::push_back with emplace_back and compiling it with C++ 11 compiler? From reading e

Answer 1

I constructed a short example that actually fails to compile when push_back is replaced by emplace_back:

#include 
struct S {
    S(double) {}
  private:
    explicit S(int) {}
};
int main() {
    std::vector().push_back(0); // OK
    std::vector().emplace_back(0); // error!
}

The call to push_back needs to convert its argument 0 from type int to type S. Since this is an implicit conversion, the explicit constructor S::S(int) is not considered, and S::S(double) is called. On the other hand, emplace_back performs direct initialization, so both S::S(double) and S::S(int) are considered. The latter is a better match, but it's private, so the program is ill-formed.