错误C4018与向量大小（）在c ++ [英] error C4018 with vector size() in c++

问题描述

当我在c ++中使用带有向量的函数.size（）时会收到警告
存储一个示例代码：

im getting a warning when i use the function .size() with vectors in c++ Heres a sample code:

vector<classname*> object;
object.push_back(new classname2);

for(int i=0;i<object.size();i++){
....}

我收到警告：

警告C4018：'<'：signed / unsigned mismatch

warning C4018: '<' : signed/unsigned mismatch

我不允许在我的最终代码中有任何错误或警告，所以我需要摆脱这个/找到一种替代方法，我该如何摆脱这一点？

I'm not allowed to have any errors or warnings in my final code so i need to get rid of this/find an alternative method, how can i get rid of this?

推荐答案

问题是，当处理签名到无符号的比较时，会有一个潜在的。如果你在32位机器上签名的 int 是4字节，那么向量的大小可能超过该类型可表示的最大数量。

The problem is that there is a potential (breaking) issue that one can incur when dealing with signed-to-unsigned comparisons. If you're on a 32-bit machine where a signed int is 4 bytes, it could be possible that the size of the vector could exceed the maximum quantity representable by that type. When that happens, you get signed overflow and consequentially Undefined Behavior.

以下是您可以使用的几种替代方法：

Here are a few alternatives you can uses:

for (std::vector<classname>::size_type i = 0; i < object.size(); ++i);

这保证是正确的，因为它是 code>返回。

This is guaranteed to be correct as it is the type the size returns.

std::vector<classname>::iterator it;

for (it = object.begin(); it != object.end(); ++it);

C ++ 11：基于范围的：

C++11: Range-based for:

for (auto& a : object)
{
     // ...
}

$ b b

std :: size_t ：

std::size_t:

for (std::size_t i = 0; i < object.size(); ++i);

如doomster在注释中所说， std :: size_t 很可能具有您的底层平台的位元大小。

As doomster said in the comments, std::size_t is likely to have the bit-size of your underlying platform.

for (unsigned int i = 0; i < object.size(); ++i);

_{注意：使用这个，你假设 返回一个32位整数。一般来说这不是一个问题，但你不能太确定;}

_{Note: By using this, you're assuming that size returns a 32-bit integer. Generally this isn't a problem, but you can't be too sure; use any of the above if you can.}

相对于代码的另一个提示是使用 unique_ptr / shared_ptr 以便于内存管理：

Another tip relative to your code is to use a vector of unique_ptr/shared_ptr to facilitate memory-management:

std::vector<std::unique_ptr<classname>> object;

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