std :: vector置换和std :: vector置换成对

Question

I have this code:

std::vector<std::pair<const std::string, int>> vec;

vec.emplace_back("a", 1); //success
vec.emplace(vec.end(), "b", 2); //compile error

vec.emplace_back(std::make_pair<const std::string, int>("c", 3));  //success
vec.emplace(vec.end(),
     std::make_pair<const std::string, int>("d", 4)); //compile error

Can you explain me why?

Answer 1

Other than std::vector::emplace_back , std::vector::emplace requires the element type meet the requirement of MoveAssignable , but std::pair<const std::string, int> has const std::string as its first element then doesn't satisfy that.

Type requirements

• T (the container's element type) must meet the requirements of MoveAssignable , MoveInsertable and EmplaceConstructible .