c++ - 奇怪的 std::vector::emplace() 编译错误

Question

我在使用 std::vector::emplace() 和 std::vector::emplace_back() 时遇到了一个奇怪的编译器错误:

#include <vector>

struct Foo {
    int bar;

    Foo(int _bar) : bar(_bar) { }
};

int main() {
    // Declaration 1
    std::vector<Foo> vec(10);

    // Declaration 2
    // std::vector<Foo> vec{};

    vec.emplace_back(1);

    return 0;
}

当我 compile这，我收到以下错误:

In file included from /usr/include/c++/6/vector:62:0,
                 from prog.cpp:2:
/usr/include/c++/6/bits/stl_construct.h: In instantiation of ‘void std::_Construct(_T1*, _Args&& ...) [with _T1 = Foo; _Args = {}]’:
/usr/include/c++/6/bits/stl_uninitialized.h:519:18:   required from ‘static _ForwardIterator std::__uninitialized_default_n_1<_TrivialValueType>::__uninit_default_n(_ForwardIterator, _Size) [with _ForwardIterator = Foo*; _Size = long unsigned int; bool _TrivialValueType = false]’
/usr/include/c++/6/bits/stl_uninitialized.h:575:20:   required from ‘_ForwardIterator std::__uninitialized_default_n(_ForwardIterator, _Size) [with _ForwardIterator = Foo*; _Size = long unsigned int]’
/usr/include/c++/6/bits/stl_uninitialized.h:637:44:   required from ‘_ForwardIterator std::__uninitialized_default_n_a(_ForwardIterator, _Size, std::allocator<_Tp>&) [with _ForwardIterator = Foo*; _Size = long unsigned int; _Tp = Foo]’
/usr/include/c++/6/bits/stl_vector.h:1309:36:   required from ‘void std::vector<_Tp, _Alloc>::_M_default_initialize(std::vector<_Tp, _Alloc>::size_type) [with _Tp = Foo; _Alloc = std::allocator<Foo>; std::vector<_Tp, _Alloc>::size_type = long unsigned int]’
/usr/include/c++/6/bits/stl_vector.h:281:30:   required from ‘std::vector<_Tp, _Alloc>::vector(std::vector<_Tp, _Alloc>::size_type, const allocator_type&) [with _Tp = Foo; _Alloc = std::allocator<Foo>; std::vector<_Tp, _Alloc>::size_type = long unsigned int; std::vector<_Tp, _Alloc>::allocator_type = std::allocator<Foo>]’
prog.cpp:11:25:   required from here
/usr/include/c++/6/bits/stl_construct.h:75:7: error: no matching function for call to ‘Foo::Foo()’
     { ::new(static_cast<void*>(__p)) _T1(std::forward<_Args>(__args)...); }
       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
prog.cpp:7:2: note: candidate: Foo::Foo(int)
  Foo(int _bar) : bar(_bar) { }
  ^~~
prog.cpp:7:2: note:   candidate expects 1 argument, 0 provided
prog.cpp:4:8: note: candidate: constexpr Foo::Foo(const Foo&)
 struct Foo {
        ^~~
prog.cpp:4:8: note:   candidate expects 1 argument, 0 provided
prog.cpp:4:8: note: candidate: constexpr Foo::Foo(Foo&&)
prog.cpp:4:8: note:   candidate expects 1 argument, 0 provided

但是，如果我注释掉声明 1 并改为使用声明 2，则代码可以正常编译。这是怎么回事？

Answer 1

您的问题不在于 vec.emplace_back(1); .由于 std::vector<Foo> vec(10); 而导致编译错误.该行试图创建一个包含 10 个默认构造元素的 vector 。由于您的类没有默认构造函数，因此您无法创建 10 个默认元素。

要使其正常工作，您需要提供它可以复制到 vector 中的类的实例。看起来像

std::vector<Foo> vec(10, Foo(whatever_number_you_want));

或者您可以只添加一个默认构造函数。

---

std::vector<Foo> vec{};不会给您带来任何问题，因为它不会尝试默认构造任何元素。空构造函数返回一个大小为 0 的 vector ，这意味着没有构造任何对象，从而避免了您未定义的默认构造函数。