#include <iostream> #include <string> class NoName {  public:      NoName(): pstring(new std::string), i(0), d(0) {            std::cout << "default ctor is called" << std::endl;      }        NoName(const NoName& other)          : pstring(new std::string(*(other.pstring))), i(other.i), d(other.d) {              // C++ 默认的copy ctor是              // pstring(other.pstring)，这是浅拷贝，是错的              std::cout << "copy ctor is called" << std::endl;          }        NoName& operator=(const NoName rhs) {          std::cout << "operator= is called" << std::endl;          pstring = new std::string;          *pstring = *(rhs.pstring);          i = rhs.i;          d = rhs.d;          // C++ 默认的operator=是          // pstring = ths.pstring是浅拷贝，是错误的          return *this;      }    private:      std::string *pstring;      int i;      double d;  };  int main() {     std::cout << std::endl <<"------test------" << std::endl;     std::cout << "NoName x, y:" << std::endl;     NoName x, y;     std::cout << std::endl;     std::cout << "NoName z(x):" << std::endl;     NoName z(x);     std::cout << std::endl;     std::cout << "x = y:" << std::endl;     x = y;        std::cout << std::endl;     return 0; }

为什么x= y这一步即调用了copy ctor又调用了operator=

------test------ NoName x, y: default ctor is called default ctor is called NoName z(x): copy ctor is called x = y: copy ctor is called operator= is called