英文:

Can an object with uninitialized fields be safely added in std::vector?

问题 {#heading}

在以下程序中，结构体 A 的默认构造函数未初始化其字段 v。然后，在一个常量表达式中，使用 A() 对象对 std::vector<A> 进行了emplace操作：

#include <vector>

struct A {
  constexpr A() noexcept {}
  int v;
};

constexpr bool f() {
    std::vector<A> as;
    as.reserve(1);
    as.emplace_back();
    return true;
}
static_assert( f() );

MSVC编译器对未初始化变量的读取发出投诉：

<source>(14): error C2131: expression did not evaluate to a constant
<source>(11): note: failure was caused by a read of an uninitialized symbol
<source>(11): note: see usage of 'A::v'

但GCC和Clang编译器都对该程序没有问题。在线演示：https://godbolt.org/z/addx11aTT

哪个编译器是正确的？ 英文:

In the following program, default constructor of struct A does not initialize its field v. Then in a constant expression, std::vector<A> is emplaced with A() object:

#include <vector>

struct A {
  constexpr A() noexcept {}
  int v;
};

constexpr bool f() {
    std::vector<A> as;
    as.reserve(1);
    as.emplace_back();
    return true;
}
static_assert( f() );

MSVC compiler complains about the reading of uninitialized variable:

<source>(14): error C2131: expression did not evaluate to a constant
<source>(11): note: failure was caused by a read of an uninitialized symbol
<source>(11): note: see usage of 'A::v'

But both GCC and Clang are fine with the program. Online demo: https://godbolt.org/z/addx11aTT

Which compiler is correct here?

答案1 {#1}

得分: 1

看起来MSVC报告了一个读取错误。显然，通过评估然后丢弃emplace_back()返回的引用形式，而不仅仅是丢弃它。MSVC在一个简化版本中展示了一个bug。请参见下文。

问题不在于向量中的分配，而是emplace_back()是一个返回表达式的方法，是对添加的元素的引用。

这也可以通过以下方式看出，它创建了一个A对象，提供了对它的引用，然后，在评估引用时，在MSVC中引发了错误。它不应该这样，正如@user17732522在评论中指出的那样。它应该被丢弃为未使用的lvalue。

struct A {
    constexpr A() noexcept {}
    int v;
};

constexpr bool f() {
    A a;
    A& b = a;
    // b;   // 取消注释这一行会导致MSVC失败
    return true;
}
static_assert(f());

int main() {}

编译正常，但如果评估引用对象b，则会出现以下错误：

Message failure was caused by a read of an uninitialized symbol

这是一个MSVC的bug。 英文:

Looks like MSVC is reporting a read error. Apparently by evaluating then discarding the returned reference form emplace_back() rather than just discarding it. MSVC does demonstrate a bug in a simplified version. See below.

The problem isn't allocation in vector, it's that the emplace_back() is a method that returns an expression, a reference to the added element.

This can be also seen by the following which creates an A object, provides a reference to it, then, when the reference is evaluated it causes an error in MSVC. It should not, as @user17732522 pointed out in comments. It should be discarded as an unused lvalue.

struct A {
    constexpr A() noexcept {}
    int v;
};

constexpr bool f() {
A a;
A\& b = a;
// b;   // uncommenting this causes MSVC to fail
return true;
}
static_assert(f());

`int main() {}
`

Compiles fine but if the b, a reference to the objects is evaluated this error ensues:

Message		failure was caused by a read of an uninitialized symbol	

This is a MSVC bug.