在C++中如何获取float[]类型数组的大小？含第三方头文件场景

Alright, let's tackle your questions one by one—this is a super common point of confusion in C++, so great call asking about it!

1. Can you get the size of a float[] array in C++? #

Yes, but it depends on whether the array is still treated as an array type (not a pointer). Here's the breakdown:

• When the array is declared in your current scope (or you have access to its full type definition), you can calculate its size using the sizeof operator:

  float my_local_arr[7];
  size_t arr_size = sizeof(my_local_arr) / sizeof(my_local_arr[0]);
  // arr_size will be 7 here
  

  This works because sizeof(my_local_arr) returns the total number of bytes the array occupies, and dividing by the size of a single float gives you the number of elements.

• However, if the array decays to a pointer (like when you pass it to a function, or assign it to a pointer variable), you can't get the original array size using sizeof. For example:

  void some_function(float arr[]) {
      // arr is actually a pointer here, not an array!
      size_t wrong_size = sizeof(arr) / sizeof(arr[0]);
      // wrong_size will be something like 2 (on 64-bit systems, since sizeof(pointer) is 8, sizeof(float) is 4)
  }
  
  int main() {
      float my_arr[7];
      some_function(my_arr); // Array decays to pointer here
      return 0;
  }
  

  In this case, arr inside some_function is just a pointer to the first element of the array, so sizeof(arr) gives the size of the pointer, not the array.

2. Can you get the size of a float[] variable from a third-party header at runtime? #

Again, this depends on how the variable is declared in the third-party header:

• If the header declares it as a fixed-size array (either explicitly or via an extern definition that references a fixed-size array), you can use the same sizeof trick as above—and since this calculation happens at compile time, the value is perfectly usable at runtime. For example, if the header has:

  // Third-party header
  extern float third_party_arr[15];
  

  Then in your code:

  size_t arr_size = sizeof(third_party_arr) / sizeof(third_party_arr[0]);
  // arr_size will be 15, and this value is available at runtime as a compile-time constant
  

• If the header declares it as a pointer (even if it's meant to point to an array), like:

  // Third-party header
  extern float* third_party_arr;
  

  You can't get the array size from the pointer alone—C++ doesn't track the size of dynamically allocated arrays automatically. You'd need the third-party library to provide an additional variable (like extern size_t third_party_arr_size;) or a function that returns the size.

• If the header has an incomplete array type declaration (like extern float third_party_arr[]; without a size), you'll need to make sure the actual definition (in the third-party library's implementation) specifies the size. If it does, the sizeof trick will work; if it's a dynamically allocated array, you're back to needing an explicit size from the library.

One last note: The sizeof calculation is a compile-time operation, but the resulting value is a constant that you can use freely at runtime (e.g., in loops, conditionals, etc.).

内容的提问来源于stack exchange，提问作者zell