为什么预处理器指令会影响我的 C++ 向量？

Question

I have a class with a block of members that are wrapped in a preprocessor directive:

class MyClass
{

... 

#ifdef MyConditionalCompilation

MyObject myObject;
std::string myString;
uint _myInt;

#endif

...

}

The class also has a member vector std::vector<int> myVector . The conundrum is as follows:

When I place the vector after the preprocessor block, thusly:

#ifdef MyConditionalCompilation

MyObject myObject;
std::string myString;
uint _myInt;

#endif

std::vector<int> myVector

The vector's address gets... lost? Moved? during runtime. I can stop at a breakpoint and see members of the vector, as well as its address via &myVector , in certain places of my code, but later during execution the vector has lost its memory address and the objects in the vector are gone.

When I move the vector above the preprocessor block, thusly:

std::vector<int> myVector

#ifdef MyConditionalCompilation

MyObject myObject;
std::string myString;
uint _myInt;

#endif

I do not experience the same issue. The vector, its address, and the values I've pushed into it are consistent across runtime.

What on earth is going on here?

For clarity, MyConditionalCompilation is true, so the code block is included in the class when it's compiled.

EDIT: For additional clarity, I've changed the conditional compilation as follows:

#define MyConditionalCompilation
#ifdef MyConditionalCompilation

MyObject myObject;
std::string myString;
uint _myInt;

#endif

So the code block is absolutely included in both of the test cases described above. I've also verified that the instance of the object is the same in example 1, where the vector "vanishes" at runtime.

Answer 1

Is your class defined in a header, perhaps? In this case make sure every file that #include s it has MyConditionalCompilation set with the same value as its implementation, because in C and C++ each .cpp / .cc file is compiled as a single, standalone translation unit.

Given that you are conditionally adding fields to MyClass its size will change, causing the relative offsets of its member fields to differ.

An instance allocated in one file compiled without MyConditionalCompilation will have myVector at a different address than one compiled with MyConditionalCompilation set to 1, and passing it to code that assumes it has a different layout will most definitely cause a runtime crash.

A common pattern that is used to avoid this issue is to ship alongside your headers a config.h header containing the configuration values used to build your library.

Answer 2

Per the recommendations of mcilloni and David Schwartz, I dug into the problem more exhaustively. The answer was unrelated to the original question, so I'll mark this resolved.