重新定义一个没有按预期工作的宏

Question

我在 C++ 中定义块，这些块位于特殊的内存区域。我想定义一个块，然后在为每个块重新定义的变量中定义下一个块的地址。

#include <iostream>

using namespace std;

#define BASE_ADDRESS 0X1000

// type a gets 100 bytes
#define TYPE_A BASE_ADDRESS
#define NEXT_FREE_BLOCK (BASE_ADDRESS + 100)
    
// type b gets 200 bytes, starting at the next free address
#define TYPE_B NEXT_FREE_BLOCK
#undef NEXT_FREE_BLOCK
#define NEXT_FREE_BLOCK (TYPE_B + 200)

// ---end of RWW memory map---

int main()
{
    cout<<"Hello free block " << NEXT_FREE_BLOCK << endl;
    return 0;
}

我查找的示例代码如下所示。但是当我尝试编译它时，我得到“由于以下错误编译失败”：

main.cpp:12:16: error: ‘NEXT_FREE_BLOCK’ was not declared in this scope
 #define TYPE_B NEXT_FREE_BLOCK
                ^
main.cpp:14:26: note: in expansion of macro ‘TYPE_B’
 #define NEXT_FREE_BLOCK (TYPE_B + 200)
                          ^~~~~~
main.cpp:20:34: note: in expansion of macro ‘NEXT_FREE_BLOCK’
     cout<<"Hello free block " << NEXT_FREE_BLOCK << endl;

                                  ^~~~~~~~~~~~~~~

如果我注释掉 'type b' 行，它会按预期运行。如果我注释掉 #undef 行，它会告诉我我已经重新定义了宏。我想在不收到错误或警告的情况下重新定义宏；有办法吗？

Answer 1

让我们看一下这个例子：

#define MYMACRO 0 //MYMACRO = 0
#define ANOTHERMACRO MYMACRO //ANOTHERMACRO = MYMACRO = 0

int main() {
    return ANOTHERMACRO;
}

还好吧？

但是如果我们这样做：

#define MYRECURSIVEMACRO 0 //MYRECURSIVEMACRO = 0

#define MYMACRO MYRECURSIVEMACRO //MYMACRO = MYRECURSIVEMACRO

#undef MYRECURSIVEMACRO // MYRECURSIVEMACRO does not exist
#define MYRECURSIVEMACRO (MYMACRO+10) //What was MYMACRO again? It was MYRECURSIVEMACRO, but now we are setting MYRECURSIVEMACRO, which right now is being defined, to itself plus 10, and now I am 100% confused

#define ANOTHERMACRO MYRECURSIVEMACRO //Now what?

int main() {
    return ANOTHERMACRO;
}

我们得到这个：

1>C:\dev\Stack Overflow\Source.cpp(520,9): error C2065: 'MYMACRO': undeclared identifier

（至少对于 MSVC++）

现在，如果我们看这个：


#define BASE_ADDRESS 0X1000 //BASE_ADDRESS = 0x1000

// type a gets 100 bytes
#define TYPE_A BASE_ADDRESS //TYPE_A = BASE_ADDRESS = 0x1000
#define NEXT_FREE_BLOCK (BASE_ADDRESS + 100) //NEXT_FREE_BLOCK = 0x1000 + 100
    
// type b gets 200 bytes, starting at the next free address
#define TYPE_B NEXT_FREE_BLOCK //TYPE_B = NEXT_FREE_BLOCK
#undef NEXT_FREE_BLOCK //NEXT_FREE_BLOCK is gone
#define NEXT_FREE_BLOCK (TYPE_B + 200) //Now what? Same problem as before!

// ---end of RWW memory map---

这就是它不起作用的原因。

重新定义一个没有按预期工作的宏

redefining a macro not working as expected

c++

preprocessor