push_back vs emplace_back in c++

Topics

• c++ STL
• c++ internals

Key differences:

1. Construction Location
   • push_back() creates the object first, then copies/moves it into the vector
   • emplace_back() constructs the object directly in the vector’s memory
2. Parameter Handling
   • push_back() takes a complete object of the container’s type
   • emplace_back() takes constructor arguments and forwards them to the object’s constructor
3. Performance
   • emplace_back() is generally more efficient as it avoids unnecessary copies/moves
   • Most noticeable with complex objects like strings or custom classes

class MyClass {
  public:
    int x;
    std::string y;
 
  public:
    MyClass(int x, std::string y) : x(x), y(y) {}
  };
 
  std::vector<MyClass> vec;
 
  vec.push_back(MyClass(42, "test")); // Creates temporary object, then moves it
  vec.emplace_back(42, "test"); // Constructs object directly in vector}
 
  cout << vec[0].x << endl;

Best practices:

• Use emplace_back() when constructing new objects
• Use push_back() when adding existing objects
• push_back() might be clearer when working with simple types like int or double

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