Defensive Copies in Modern C++

Modern C++은 이 문제를 다르게 해결합니다:

std::string s1 = "Cpp";
std::string s2 = s1;  // Duplicate the data in s1.
  • s1의 힙 데이터는 복제되고, s2는 독립적인 복사본을 얻습니다.
  • s1 와 s2의 스코프가 종료되면 각각의 메모리가 해제됩니다.

복사 전:

StackHeaps1ptrCpplen3capacity3

복사 후:

StackHeaps1ptrCpplen3capacity3s2ptrCpplen3capacity3

키 포인트:

  • C++ has made a slightly different choice than Rust. Because = copies data, the string data has to be cloned. Otherwise we would get a double-free when either string goes out of scope.

  • C++ also has std::move, which is used to indicate when a value may be moved from. If the example had been s2 = std::move(s1), no heap allocation would take place. After the move, s1 would be in a valid but unspecified state. Unlike Rust, the programmer is allowed to keep using s1.

  • Unlike Rust, = in C++ can run arbitrary code as determined by the type which is being copied or moved.