c++ - std::basic_string 每次都会破坏空终止吗？

Question

最近，我将我的编译器从 gcc-4.3.x 更新到 gcc-7.x，然后我的一位测试人员遇到了断言异常。

代码如下:

struct data {
    data() : _c(0) { ++CREATED; std::cout<<"data  CREATED +1"<<_c<<" addres: "<<&_c<<std::endl;}
    data(char c) : _c(c) { ++CREATED; std::cout<<"data  with C CREATED +1"<<_c<<std::endl;}
    data(const data& rhs) : _c(rhs._c) { ++COPIED; }
    ~data() { ++DESTROYED; std::cout<<"data  DESTROYED +1"<<_c<<std::endl;}

    char _c;

    static size_t CREATED;
    static size_t COPIED;
    static size_t DESTROYED;
};

size_t data::CREATED   = 0;
size_t data::COPIED    = 0;
size_t data::DESTROYED = 0;

void testStringReferenceCopiable() {
    typedef std::basic_string<data> data_str;
    std::cout<<"1"<<std::endl;
    data d[] = {'a', 'b', 'c'};
    std::cout<<"2"<<std::endl;
    data_str s( &d[0], 3 );
    std::cout<<"3"<<std::endl;
    data_str s2 = s;
    std::cout<<"4"<<std::endl;
    data_str s3;
    std::cout<<"5"<<std::endl;
    s3 = s;
}

对于 gcc7.x 输出如下:

1
data(char c) CREATED +1a
data(char c) CREATED +1b
data(char c) CREATED +1c
2
data()  CREATED +1
~data()  DESTROYED +1
3
data()  CREATED +1
~data()  DESTROYED +1
4
data()  CREATED +1
~data()  DESTROYED +1
5
data()  CREATED +1
~data()  DESTROYED +1
~data()  DESTROYED +1c
~data()  DESTROYED +1b
~data()  DESTROYED +1a

对于 gcc4.3.x 版本，输出是:

data  CREATED +1
1
data  with C CREATED +1a
data  with C CREATED +1b
data  with C CREATED +1c
2
3
4
5
data  DESTROYED +1c
data  DESTROYED +1b
data  DESTROYED +1a

data  DESTROYED +1

基本上我能理解为什么它每次都调用 Construct of data() ，可能是因为 null 终止符。但是我不明白为什么它每次都调用数据的析构方法。谁能给我一个答案？

谢谢!

Answer 1

GCC 5 更改了 std::basic_string 以符合 C++11 中引入的几个新要求。

您注意到的变化是 GCC 5 中 std::basic_string 放弃了“写时复制”。也就是说，同一字符串的拷贝必须不同，而不仅仅是引用-计数。