std :: vector插入，不知道元素的类型 [英] std::vector insert without knowing the type of the elements

问题描述

假设我有一个采用各种向量的模板化函数（但是由于各种原因，我无法在template参数中提及这一点）。这是我要执行的操作：在不知道其类型的情况下，在特定位置插入新的默认构造元素：

Suppose I have a templated function that takes various kinds of vectors (but for various reasons I can't mention this in the template parameter). Here's what I'm trying to do: insert a new, default constructed element at a specific spot, without knowing its type:

template <typename T>
void foo(T* v) {
  v->insert(v->begin() + 5, decltype(v->at(0))());
}

这行不通，但让您了解我的意思试图做。我还尝试使用 std :: vector 中的 value_type ，但是我也遇到了问题。有什么想法可以解决这个问题吗？

This doesn't work, but gives you an idea of what I'm trying to do. I also tried to use value_type from std::vector but I ran into problems there as well. Any ideas how to solve this problem?

推荐答案

回避整个命名类型业务：

Sidestep the whole "name the type" business:

v->emplace(v->begin() + 5);

或

v->insert(v->begin() + 5, {});

由于 decltype（v-> at （0））是引用类型。如果正确使用 value_type 应该可以工作，但是却看不到自己在做什么，我不能说这是怎么回事。

Your current version doesn't work because decltype(v->at(0)) is a reference type. value_type should work if you use it correctly, but without seeing what you are doing I can't say what's wrong with it.

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