为什么我的 C++ 程序在使用 NULL 对象时默默退出？

Question

string n = NULL;

cout << "blah: " << n << " foo!" << endl; // never printed

cout << "BLAH" << endl; // never printed

cerr << "FOO" << endl; // never printed

I understand assigning or appending something to NULL is not right. But silently dying is not good either. How should I handle / debug those situations? And why is the program dying?

Answer 1

Your program is free to do whatever the implementation of std::string you have choses to. std::string has a constructor taking a const char* s . However, this involves:

Constructs the string with the contents initialized with a copy of the null-terminated character string pointed to by s . The length of the string is determined by the first null character. The behavior is undefined if s does not point at an array of at least Traits::length(s)+1 elements of CharT , including the case when s is a null pointer.

(emphasis mine)

_{Via cppreference}

Answer 2

You should just leave n uninitialized instead, assigning it to NULL in C++11 can lead to undefined behavior. There should not be an instance in which you need to assign to NULL.