Move constant object without compiler warning

Question

I tested the following code using Visual Studio 2017 version 15.3.1.

v.push_back(std::move(str1)) works as expected. It moves the contents of str1 into the vector.

str2 is a constant string. Since a constant string cannot be modified after it is created, I was expecting that the v.push_back(std::move(str2)) statement would result in a compiler warning. However, to my surprise there was no compiler warning. After stepped into it, I found that the overload of push_back(const T&) was actually called. The std::move in std::move(str2) seems has no effect.

My question: Should a compiler warning be given for trying to move a constant object?

// Compiled with Visual Studio 2017 version 15.3.1
std::vector<std::string> v;

std::string str1 = "string 1";
v.push_back(std::move(str1));
// Call push_back(T&&). The contents of str1 is moved into the vector.
// This is less expensive, but str1 is now valid but unspecified.

const std::string str2 = "string 2";
v.push_back(std::move(str2));
// Call push_back(const T&). A copy of str2 is added into the vector.
// str2 itself is unchanged.

Answer 1

No. Remember, std::move doesn't move anything, it is a glorified cast through remove_reference_t. Since in your case it is casted to const rvalue reference (as std::remove_reference_t<const T> is const T), it doesn't bind to rvalue reference overload push_back(T&& ), and instead binds to const lvalue reference one - push_back(const T& ).