在C ++代码中将size_t变量与-1(最大大小值)进行比较 [英] Comparison size_t variable with -1 (maximum size value) in c++ code

问题描述

我正在重构一个库，并试图摆脱许多gcc警告.这些警告的大部分与有符号/无符号比较有关，并且与 size_t 的用法有关.该库可在64位Linux系统上使用.

程序员使用-1作为特殊值，类似于 std :: string :: npos .库中很多地方的代码如下所示:

  class AnnotationBase{上市:size_t m_offset = -1;size_t m_length = -1;}...AnnotationBase foo(const std :: string& text，const AnnotationBase& annot){AnnotationBase newAnnot;//做一些注释和文字...if(newAnnot.m_offset == -1){//做其他工作...}返回newAnnot;} 

问题在于，由于有符号/无符号比较，gcc在 if(newAnnot.m_offset == -1)行上生成了警告:

 警告:有符号和无符号整数表达式[-Wsign-compare]之间的比较" 

在没有警告的情况下将C ++中的 size_t 变量与最大值(-1)比较的正确方法是什么?由于此表达式的复杂性和长度，像 if(newAnnot.m_offset == std :: numeric_limits< size_t> :: max())这样操作非常不方便.

是使用C样式定义的值 SIZE_MAX 的好方法，还是创建自己的常量，如 namesapce libling {const NONE = std :: numeric_limits< size_t> ::最大限度();} (创建新常量会在不同的库和名称空间中产生许多类似的常量，例如 libling :: NONE ， libother :: UNKNOWN ， liblongnamesapcename ::什么)?

解决方案

您可以执行 std :: string 的操作，并定义 static const size_t AnnotationBase :: npos = -1 .然后将其用作比较.我不会认为每个库都有一个常数是一个问题，但是如果要避免这种情况，可以直接使用 std :: string :: npos (尽管这样会使代码更严格).

I'm refactoring a library and trying to get rid of many gcc warnings. The big part of these warning are about signed / unsigned comparison and are related to the usage of size_t. The library works on 64 bit Linux systems.

A programmer used -1 as the special value similar to std::string::npos. There are many places in the library where code looks like this:

class AnnotationBase
{
    public:
        size_t m_offset = -1;
        size_t m_length = -1;

}

...

AnnotationBase foo(const std::string& text, const AnnotationBase& annot)
{
    AnnotationBase newAnnot;

    // do some job with annotations and text
    ...

    if(newAnnot.m_offset == -1)
    {
        // do some other job
        ...
    }

    return newAnnot;
}

The problem lies in the warning that gcc generates on the line if(newAnnot.m_offset == -1) because of a signed / unsigned comparison:

"warning: comparison between signed and unsigned integer expressions [-Wsign-compare]"

What is the proper way to compare size_t variable in C++ with the maximum value (-1) without warning? Doing this like if(newAnnot.m_offset == std::numeric_limits<size_t>::max()) is very inconvenient due to complexity and length of this expression.

Is it a good way to use C-style defined value SIZE_MAX or it is better to create own constant like namesapce libling { const NONE = std::numeric_limits<size_t>::max(); } (Creating new constant leads to many similar constants in different libraries and namespaces like libling::NONE, libother::UNKNOWN, liblongnamesapcename::NOTHING)?

解决方案

You could do what std::string does and define a static const size_t AnnotationBase::npos = -1. Then use that in comparisons as a convention. I wouldn't consider one constant per library a problem, but if you want to avoid that, you can use std::string::npos directly (that makes the code more rigid though).

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