c++ 库中的常量；不起作用

发布于 2024-07-19 11:41:53 字数 255 浏览 6 评论 0原文

任何人都知道为什么当我尝试包含具有以下声明的库时这不起作用：

namespace wincabase
{
  const char* SOMESTRING = "xx";
}

虽然这完全没问题：

namespace wincabase
{
  const int X = 30;
}

当我链接库时，我在第一种情况下收到 gcc 的“多个定义”错误。谢谢！

原文

anyone knows why this does not work when I try to include a library with the following declarations:

namespace wincabase
{
  const char* SOMESTRING = "xx";
}

While this is perfectly fine:

namespace wincabase
{
  const int X = 30;
}

I get a "multiple definitions" error with gcc for the first case when I link the lib. Thanks!

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终弃我 2024-07-26 11:41:53

const char* 表示指向 const char 的指针。这意味着指针本身不是常量。

因此它是一个普通变量，因此您需要

extern const char* SOMESTRING;

在头文件和

const char* SOMESTRING = "xx";

库的一个编译单元中使用。

或者，如果它是指向 const char 的 const 指针，那么您应该使用：

const char* const SOMESTRING = "xx";

const char* means pointer to const char. This means the pointer itself is not constant.

Hence it's a normal variable, so you'd need to use

extern const char* SOMESTRING;

in the header file, and

const char* SOMESTRING = "xx";

in one compilation unit of the library.

Alternatively, if it's meant to be a const pointer to a const char, then you should use:

const char* const SOMESTRING = "xx";

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森林散布 2024-07-26 11:41:53

您将指针声明为 const，然后将其指向编译单元中定义的字符串文字，因此如果您在头文件中使用它，则将复制字符串文字。您需要做的是在头文件中声明指针，并在库的源文件中定义字符串。

标题：

extern const char* SOMESTRING;

在库中的某些源文件中：

const char* SOMESTRING = "xx";

You're declaring the pointer as const, and then pointing it to a string literal defined in the compilation unit, so you'd be duplicating the string literal if you used this in a header file. What you need to do is declare pointer in the header file, and define the string in a source file in the library.

Header:

extern const char* SOMESTRING;

In some source file in the library:

const char* SOMESTRING = "xx";

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暮光沉寂 2024-07-26 11:41:53

除了 Tobi 指出的方法之外：

const char* const SOMESTRING = "xx";

另一种选择是将其声明为 const 字符数组：

const char SOMESTRING[] = "xx";

这种方法可能为编译器提供额外的优化机会，例如将字符串放入生成的二进制文件的只读部分；尽管可以想象，编译器可能能够使用第一种方法执行类似的优化。

Besides the approach Tobi pointed out:

const char* const SOMESTRING = "xx";

another alternative is to declare it as a const character array:

const char SOMESTRING[] = "xx";

This approach potentially provides the compiler with additional optimization opportunities, such as placing the string in the read-only section of the resulting binary; although it's conceivable the compiler may be able to perform similar optimizations with the first approach.

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樱＆纷飞 2024-07-26 11:41:53

您需要单独声明和定义它们：

Plop.h
======
namespace wincabase
{
   extern const char* SOMESTRING;  // declare
}

Plop.cpp
========
const char* wincabase::SOMESTRING = "xx"; // define

You need to declare and define them seprately:

Plop.h
======
namespace wincabase
{
   extern const char* SOMESTRING;  // declare
}

Plop.cpp
========
const char* wincabase::SOMESTRING = "xx"; // define

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