Unknown Bounded Array

有两个文件,A file is a declaration of an array,The other is the definition of the array.If the definition of the array changes,For example, it has become contained5个元素的数组,Then the associated statement must also be changed.Once there are more files, some files will be forgotten to be modified,Unexpected problems will occur.

int array[4] = {
     1, 2, 3, 4 };

#include<iostream>
extern int array[4];

int main()
{
    
	……
	return 0;
}

Some people come up with a solution,declared as a pointer.Because arrays are implicitly type converted to pointers,But this workaround gives a segfault（如果对arrayPointer dereference words）,Let's look at the program first,By the way, the address of each element is printed out：

#include<iostream>

int array[4] = {
     1, 2, 3, 4 };
void func()
{
    
	std::cout << array << std::endl;
}

#include<iostream>

void func();
extern int* array;

int main()
{
    
	func();
	//std::cout << array[1] << std::endl;
	std::cout << array << std::endl;
	return 0;
}

很显然,Only the first address matchesC++The address form of the language.那么为什么会出现这种问题呢？

First of all this is not a compile error,Because each file is compiled separately without any problem;Second is not a link problem（Types are a compiler concept,There is no type in the link,只有符号）,array符号一致;So it is a runtime error.因为指针是64位的,So it will treat the first two elements of the array as pointers（due to endianness）,Directly translated addresses are invalid,So dereferencing will segfault.

正确的做法是使用Unknown Bounded Array（C++The specialized term is called incomplete class）：

#include<iostream>

int array[4] = {
     1, 2, 3, 4 };
void func()
{
    
	std::cout << array << std::endl;
}

#include<iostream>

void func();
extern int array[];//Statements can be written like this,But definitions can't

int main()
{
    
	func();
	std::cout << array << std::endl;
	return 0;
}