向量特定语法的向量 [英] Vector of Vector specific syntax

问题描述

我通常使用以下语法声明（并定义）向量的向量

I usually declare (and define) my vector of vectors with the following syntax,

vector<vector<int>> graph = *new vector<vector<int>>(n, *new vector<int>(n, 0));

此处n已定义。这通常在大多数编译器中都可以正常工作。但是几天前，我尝试将源文件导入另一个系统，并且它充满了编译错误，例如，

'>>'之后的预期主表达式

我不记得确切的错误，但确实知道在两侧的>和>之间添加空格后，该错误已消除。即

Here n is already defined. And this usually works fine in most compilers. But a few days ago I tried importing the source file to another system, and it was filled with compilation errors something like,
"Expected primary expression after '>>' "
I do not remember the exact errors, but do know that after adding spaces between '>' and '>' on both sides the error was removed. i.e.

vector<vector<int> > graph = *new vector<vector<int> >(n, *new vector<int>(n, 0));

我知道语法要求我们在两个'>之间添加空格，但是我想知道编译器之间是否有区别，因为两者都使用c ++ 11甚至相同的IDE。我使用这种语法已经很长时间了，对每个语法进行编辑将非常令人沮丧。知道使用哪个编译器会更容易。

I know that the syntax requires us to add spaces between both '>'s, but I would like to know if there is any difference between the compilers, because both used c++11 and even the same IDEs. I have used this syntax for very long and it would be very frustrating to edit each one. It would be easier to know which compilers it works on.

推荐答案

我将为您提供两部分的答案：

I'll give you a two-part answer:

vector<...> v = *new vector<...>(...)

基本上，您不应该

Vector会执行自己的内存管理，因此不需要原始的 new 。另外，在这一行中，您为堆上的向量分配内存（仅12或24个字节，具体取决于您的系统），然后将此向量分配给向量 v 在堆栈上（可能涉及整个向量内容的副本）。堆上的向量永远不会被删除，因此内存会泄漏。

Basically, you shouldn't do this.
Vector does its own memory management, so there is no need for raw new. Also, in this line you allocate memory for a vector on the heap (which is only 12 or 24 bytes, depending on your system), then assign this vector to the vector v on the stack (which might involve a copy of the contents of the whole vector). The vector on the heap is never deleted and thus the memory is leaked.

更好：

vector<vector<int>> graph = vector<vector<int>>(n, vector<int>(n, 0));

或者只是

vector<vector<int>> graph(n, vector<int>(n, 0));

现在您的原始问题的答案是：C ++标准开始允许> > 关闭从C ++ 11开始的嵌套模板（请参见相关问题），因此您需要将编译器配置为至少使用C ++ 11标准（通常使用-std = c ++ 11标志）。但是，我所知道的几乎所有最新的编译器都已默认使用该标准。

Now the answer to your original question: The C++ standard started allowing >> to close nested templates starting from C++11 (see this related question), so you need to configure your compiler to use at least the C++11 standard (normally using the -std=c++11 flag). However, nearly all recent compilers I know already use this standard by default.

要获取更详细的答案，您需要告诉我们使用的是哪个IDE和编译器。

For a more detailed answer you would need to tell us which IDE and compiler you use.

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