如何计算外部阵列的大小？

Question

Does anyone know how to calculate the size of an array from an external file?

Consider:

Data.c

 float arr[]={1.4, 2.3, 7.6, 4.8, 3.3}; 

Main.c

 #include <stdio.h> int main() { extern float arr[]; //... } 

Then how do I go from there? I've tried using sizeof and size_t but there are still errors for both methods.

Answer 1

In your main.c file the only information the compiler has is extern float arr[]; and this doesn't include a size.

So either you try to define a function inside Data.c which returns sizeof(arr) (in this compilation unit the size should be known) or, as you tagged the question c++, you use a std::array for fixed size or a std::vector for variable size.

Answer 2

The problem is that extern float arr[] is an incomplete type , it only says that "elsewhere, there is an array called arr of unknown size". You can only use sizeof if the size is known, for example on extern float arr[5] .

However, you should not write programs like this. Using extern on non-constant variables is always an indication of bad program design.

Instead, you should use private encapsulation and all access to variables should be done through setter/getter functions. Similarly, you can create a function get_size() which returns the array size.

Answer 3

The short answer is, you can't do what you want. When you say

extern float arr[];

the compiler simply doesn't have any way of getting the size of that external array.

The simple fix is to use a second global variable that carries the size of the array:

Data.c

float arr[]={1.4, 2.3, 7.6, 4.8, 3.3};
int arrsz = sizeof(arr) / sizeof(arr[0]);

Main.c

extern float arr[];
extern int arrsz;

For extra credit, use size_t arrsz instead of int .

As some other answers have pointed out, though, global variables like this are poor style, especially in "real" programs. For some higher-style, less-simple fixes, see those other answers.