什么时候在函数调用中发生临时对象的破坏？

Question

此代码编译并执行.我知道在第一种情况下我们有未定义的行为.但在第二种情况下究竟发生了什么？

#include <string>
#include <iostream>
#include <cstdio>

std::string foo() {
    return "HELLO";
}

void bar(const char *p) {
    std::printf("%s\n", p);
}


int main() {

    // FIRST CASE:
    // I know this is bad, because after the assignment
    // the variable returned by foo() is destroyed and we
    // have a bad reference.
    const std::string &s = foo(); 
    bar(s.c_str());


    // SECOND CASE:
    // But what about that ? I don't know exactly if the 
    // object is alive after the call to c_str() 
    bar(foo().c_str());

    return 0;
}

GCC输出在两种情况下都是"HELLO",但我认为这是因为它没有清理原始内存.

在第二种情况下,临时对象究竟被销毁了吗？

Answer 1

实际上,这两种情况都没问题.

在第一种情况下,将临时绑定到const引用会将其生命周期延长到s.因此在main退出之前不会被销毁.

在第二种情况下,临时在包含它的完整表达式结束后被销毁.在这种情况下,它是函数调用.如果你将C字符串存储在任何已经过时的状态bar,然后尝试访问它,那么你应该是一个具有未定义行为的日期.

Answer 2

这两种情况都有明确的定义。要查看有问题的情况，请将结果存储c_str()到std::string被破坏之后：

#include <string>
#include <cstdio>

std::string foo() {
    return "HELLO";
}

void bar(const char *p) {
    std::printf("%s\n", p);
}

int main() {
    {
        // FIRST CASE:
        // This is okay, because the reference is const, so the object is alive
        // until s goes out of scope.
        const std::string &s = foo();
        bar(s.c_str());
    }

    {
        // VARIANT FIRST CASE:
        // This is bad; the pointer is dangling
        const char *s = foo().c_str();
        bar(s);
    }

    {
        // SECOND CASE:
        // Is the object still alive after the call to c_str()?  Yes, it's alive
        // until after bar() has returned.
        bar(foo().c_str());
    }

    return 0;
}

当我在 Valgrind 下运行它时，它仅针对变体情况（带有const char *s）给出错误：

==9981== Invalid read of size 1
==9981==    at 0x4C2E0E2: strlen (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==9981==    by 0x543EC7B: puts (ioputs.c:36)
==9981==    by 0x400937: bar(char const*) (37946437.cpp:9)
==9981==    by 0x4009AA: main (37946437.cpp:25)
==9981==  Address 0x5aabcf8 is 24 bytes inside a block of size 30 free'd
==9981==    at 0x4C2C2BC: operator delete(void*) (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==9981==    by 0x4F058FD: std::basic_string<char, std::char_traits<char>, std::allocator<char> >::~basic_string() (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.22)
==9981==    by 0x40099E: main (37946437.cpp:24)