不能在向量向量上 emplace_back() 一个带支撑的初始值设定项 [英] Cannot emplace_back() a braced initializer on a vector of vectors

问题描述

这与我之前提出的关于在成对向量上使用 emplace_back 的问题有些相关.emplace_back() vs push_back 插入一对时std::vector

This is somewhat related to a previous question I asked regarding using emplace_back on a vector of pairs. emplace_back() vs push_back when inserting a pair into std::vector

现在我的问题是关于在向量向量上使用 emplace_back.

Now my question pertains to using emplace_back on a vector of vectors.

这是我用评论质疑的代码

Here is the code that I am questioning about with comments

std::vector<std::vector<int>> matrix;

matrix.emplace_back({1,2,3}); //doesn't compile

matrix.emplace_back(1,2,3); //doesn't compile

matrix.push_back({1,2,3}); //works and does what is expected (insert a vector made of {1,2,3} into matrix);

matrix.emplace_back(std::vector<int>{1,2,3});   //works but 
//defeats the purpose of using emplace_back since this makes a copy
//and is thus equivalent to push_back in this case?

matrix.emplace_back(3,2) //this compiles, 
//but it seems to insert a vector of size 3 made of 2s into the matrix. 
//not actually sure why it does this

所以，由此看来，matrix.emplace_back(std::vector{1,2,3}); 似乎是使用 std:: 的唯一正确方法vector<T>::emplace_back 在向量的向量上，但这似乎比 push_back 没有任何优势.我对这个问题的理解是否正确?

So, from this, matrix.emplace_back(std::vector<int>{1,2,3}); seems to be the only correct way to use std::vector<T>::emplace_back on a vector of vectors, but this seems to offer no advantages over push_back. Is my understanding on this matter correct?

另外，有人能解释一下为什么 matrix.emplace_back(3,2) 将一个由 2s 组成的大小为 3 的向量插入矩阵吗?

Also, could someone explain why matrix.emplace_back(3,2) is inserting a vector of size 3 made of 2s into the matrix?

推荐答案

{1, 2, 3} 在这种情况下不能推导出initializer_list(这是您想要使用的 vector 构造函数所期望的.)所以您需要帮助它:

The {1, 2, 3} cannot be deduced to initializer_list<int> in this case (which is what the vector<int> constructor you want to use expects.) So you need to help it along a bit:

matrix.emplace_back(initializer_list<int>{1, 2, 3});

当使用 push_back() 时，这不是必需的.我不知道确切的细节，但 emplace_back() 是一个函数模板，而 push_back() 不是.模板的推导规则是不同的(更严格.)并且花括号初始化器没有类型.正因为如此，它有自己的关于类型推导如何工作的特殊规则.

This is not required when using push_back(). I don't know the exact details, but emplace_back() is a function template while push_back() is not. Deduction rules for templates are different (way more strict.) And a braced initializer has no type. Because of that, it comes with its own special rules on how type deduction works.

至于高效，这个:

matrix.emplace_back(vector<int>{1, 2, 3});

构造两个向量.matrix 中的空向量，以及传递的临时值.临时文件被移动到空向量中.所以真的没那么糟糕.

constructs two vectors. An empty vector in matrix, and the passed temporary. The temporary is moved into the empty vector. So it's not that bad really.

然而，这:

matrix.emplace_back(initializer_list<int>{1, 2, 3});

只构造一个向量，使用接受初始化列表的构造函数.请注意，这里没有创建额外"的 initializer_list 对象.在使用花括号初始化创建任何向量时，无论如何都会创建这样的对象:

Only constructs one vector, using the constructor that accepts an initializer_list. Note that there's no "extra" initializer_list object being created here. Such an object is going to be created anyway when creating any vector using braced initialization:

vector<int> vec{1, 2, 3};

这也创建了一个 initializer_list 对象，因为这是向量构造函数所需要的.

This also creates an initializer_list object, because that's what the vector constructor takes.

至于为什么 emplace_back(2,3) 起作用，那是因为有一个接受大小和值的向量构造函数.

As for why emplace_back(2,3) works, that's because there's a vector constructor that takes a size and a value.

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