如何从 2 个不同的文件访问结构数组？

Question

How would I go about doing something like this?

header.h

 struct myStruct;
 extern myStruct array[];

main.cpp

#include "header.h"
struct myStruct{
int a = 0;
};
myStruct array[5];
array[0].a = 1;

file2.cpp

#include "header.h"
std::cout << array[0].a;

Answer 1

The size and layout of myStruct needs to be known everythere you try to access its contents, so you must either define it inside of the.h file, or only access it using opaque pointers and dedicated functions.

Also, using C arrays in C++ is somewhat outdated. You should use STL containers instead, such as std::array<T, N> or std::vector<T> .

Answer 2

Well, first of all you should put struct declarations in the header files.

Second, you can't put assignments to arrays or std::cout statements just loose in files. I'm assuming you did this for short code and not because you didn't know it needed to be in a function, but I'm pointing it out just in case.

Third, struct declarations require a semicolon

What you would do is have a header file:

header.hpp:

#pragma once

struct MyStruct {
    int a = 0;
};

extern MyStruct array[5];

The extern here means that the data is somewhere else. So let's make the somewhere else.

header.cpp:

#include "header.hpp"

MyStruct array[5];

main.cpp:

#include "header.hpp"

void someFunction() {
    ...
    array[0].a = 1;
    ...
}

file2.cpp:

#include <iostream>
#include "header.hpp"

void someOtherFunction() {
    ...
    std::cout << array[0].a;
    ...
}

Answer 3

How would I go about doing something like this?

You could go about doing something like what you did by doing something like what you did, except fixing the bugs in your attempt. Here is the list of bugs:

• You failed to end the definition of myStruct with a semicolon.
• .c suffix is conventionally for C source files while you're writing C++. Your compiler will likely be confused unless you provide some unconventional options to the compiler.
• You use cout which you forgot to declare. Or did you intend to use std::cout ? In that case you must include <iostream> in order to use it, and use the appropriate namespace.
• In file2.c , you use the class myStruct in a way that depends on the definition of myStruct without providing that definition.
• array[0].a = 1; and cout << array[0].a; are expression statements. Expression statements may not be in namespace scope, and must be within a block scope. In your program, those expression statements are in the namespace scope which makes the program ill-formed.