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std :: size_t vs size_t vs std :: string :: size_type

[英]std::size_t vs size_t vs std::string::size_type

原文 2011-12-14 16:19:43 3 3 c++

Where does size_t come from when I don't have anything included? 当我没有包含任何东西时， size_t来自哪里？
Is it reasonable to always assume size_t == std::size_t ? 总是假设size_t == std::size_t是否合理？
When should I use the size_type in std containers ( string::size_type , vector<T>::size_type , etc)? 我什么时候应该在std容器中使用size_type （ string::size_type ， vector<T>::size_type等）？

3 个解决方案

Where does size_t come from when I don't have anything included in an empty project? 当我没有包含在空项目中的任何内容时，size_t来自哪里？

If you don't have anything included, then you can't use size_t . 如果您没有包含任何内容，则不能使用size_t 。 It's defined in <stddef.h> (and perhaps also in <cstddef> , if your version of that header puts the definitions in the global namespace as well as std ). 它在<stddef.h>定义（也可能在<cstddef> ，如果您的头版本将定义放在全局命名空间和std ）。

Is it reasonable to always assume size_t == std::size_t? 总是假设size_t == std :: size_t是否合理？

Yes. 是。 All types and functions defined by the C library are included in the std namespace, as long as you include the appropriate C++ header (eg <cstddef> rather than <stddef.h> ) 只要包含适当的C ++标头（例如<cstddef>而不是<stddef.h> ），C库定义的所有类型和函数都包含在std命名空间中。

When should I use std:: _ ::size_type? 我什么时候应该使用std :: _ :: size_type？

Do you mean the size_type types defined in some standard classes and templates such as vector ? 你的意思是在一些标准类和模板中定义的size_type类型，例如vector ？ You could use those when using those classes if you like. 如果您愿意，可以在使用这些类时使用它们。 In most cases, you'll know that it's the same as size_t , so you might as well use that and save a bit of typing. 在大多数情况下，你会知道它与size_t相同，所以你不妨使用它并节省一些输入。 If you're writing generic code, where you don't know what the class is, then it's better to use size_type in case it's not compatible with size_t . 如果您正在编写通用代码，而您不知道该类是什么，那么最好使用size_type ，以防它与size_t不兼容。

For example, you might want to write a container designed to hold more items than can be represented by size_t . 例如，您可能希望编写一个容器，用于容纳比size_t表示的更多的项目。 You might use some kind of big number type to represent the container's size, which isn't convertible to size_t . 您可以使用某种大数字类型来表示容器的大小，该大小不可转换为size_t 。 In that case, code like size_t s = c.size() would fail to compile - you'd need to use Container::size_type instead. 在这种情况下，像size_t s = c.size()这样的代码将无法编译 - 您需要使用Container::size_type 。

Where does size_t come from when I don't have anything included in an empty project? 当我没有包含在空项目中的任何内容时， size_t来自哪里？

size_t is a base unsigned integer memsize-type defined in the standard library of C/C++ languages. size_t是在C / C ++语言的标准库中定义的基本无符号整数memsize-type。 It is defined in "stddef.h" for C and in <cstddef> for C++ . 它在C "stddef.h"定义，在C++ <cstddef>定义。

Types defined by the header file "stddef.h" are located in the global namespace while <cstddef> header places the size_t type in the namespace std . 头文件"stddef.h"定义的类型位于全局名称空间中，而<cstddef>标头将size_t类型放在名称空间std 。

"stddef.h" from C is included into C++ for compatibility, and hence the type can be found in both the global namespace ( ::size_t ) and the std namespace ( std::size_t ). 来自C的"stddef.h"包含在C ++中以实现兼容性，因此可以在全局命名空间（ ::size_t ）和std命名空间（ std::size_t ）中找到该类型。