将指针从C传递到C ++，反之亦然 [英] passing pointers from C to C++ and vice versa

问题描述

有没有任何提示可以给我关于传递指针到结构，双精度，函数，从一个C程序到一个C ++库和回来？

Is there any tips one can give me about passing pointers to structs, doubles, functions, ... from a C program to a C++ library and back?

推荐答案

假设你在两个不同的库静态或动态编译这些（在Linux上的DLL共享库和其他* nix变体）我最关心的问题如下：

Assuming you're coding these in two different libraries static or dynamic (DLLs on windows shared libraries on Linux and other *nix variants) The biggest concerns I have are as follows:

1. 它们是使用相同的编译器编译的。虽然如果所有C ++导出都以C风格的命名约定导出，则不需要这样做，但是需要在两个C ++模块之间对C ++和C ++调用类实例。这是必要的，因为不同的编译器mangle C ++导出不同。

1. They are compiled with the same compiler. While this isn't necessary if all C++ exports are exported with a C-style naming convention it is necessary for C++ to C++ calls to class instances between the two C++ modules. This is necessary due to how different compilers mangle C++ exports differently.

不要将C ++类作为C结构。它们在封面下是不一样的，即使字段的布局是相同的。 C ++类有一个v表，如果他们有任何虚拟成员;这个v表允许正确调用继承或基类方法。

Do not cast a C++ class as a C struct. They aren't the same under the covers, even if the layout of fields are the same. C++ classes have a "v-table" if they have any virtual members; this v-table allows the proper calling of inherited or base class methods.

C到C或C ++到C ++以及C到C ++ 。确保两者对输出库使用相同的字节对齐。您只能通过阅读您的编译器或开发环境文档来确定这一点。

This is true of C to C or C++ to C++ as well as C to C++. Ensure both use the same byte alignment for the output library. You can only determine this by reading your compiler or development environments documentation.

不要将malloc / free与new / delete混合。更具体地说，不分配具有空闲的新的和空闲存储器的存储器，反之亦然。许多编译器和操作系统在两者之间处理不同的内存管理。

Don't mix malloc/free with new/delete. More specifically don't allocate memory with new and free memory with "free" and vice versa. Many compilers and operating systems handle memory management differently between the two.

传递函数指针：只要它们作为extern C这应该很好。 （你需要引用你的编译器文档，如何确定何时一个头被编译为C或C ++来保持在一个文件，或者你将需要在每个项目中相同的函数声明的两个单独的副本 - I推荐第一个）

Passing function pointers: So long as they are exposed to/from C++ as ''extern "C"'' this should be fine. (You'll either need to reference your compilers documentation on how to determine when a header is being compiled as C or C++ to maintain this in one file, or you will need two separate copies of the same function declaration in each project -- I recommend the first)

传递双重：这是C和C ++中的内置类型，应该处理相同。

Passing doubles: This is a built-in type in both C and C++ and should be handled the same.

如果您必须与C函数共享一个C ++对象的实例，并从C代码中对其进行操作，则公开一组C语言导出的辅助函数，它们调用相应的方法对C ++对象。纯C代码无法正确调用C ++对象上的方法。

If you must share an instance of a C++ object with a C function, and act on it from within C code, expose a set of C-exported helper functions which call the appropriate methods on the C++ object. Pure C code cannot properly call methods on C++ objects.

    Pseudocode-ish Example:
    // C++ class
    class foo {
       public:
           void DoIt();
    };

    // export helper declarations
    extern "C" void call_doit(foo* pFoo);
    extern "C" foo* allocate_foo();
    extern "C" deallocate_foo(foo* pFoo);


    // implementation
    void call_doit(foo* pFoo)
    {
        pFoo->DoIt();
    }

    foo* allocate_foo()
    {
        return new foo();
    }

    deallocate_foo(foo* pFoo)
    {
       delete pFoo;
    }

    // c consumer
    void main()
    {
        foo* pFoo= allocate_foo();
        call_doit(pFoo);
        dealocate_foo(pFoo);
    }

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