C++中字符串数组指针的正确定义方式及写法差异疑问

Great question—let's break this down step by step to clarify each part of your inquiry.

1. std::string* str_arr_p[]; 是否合法？是否必须指定数组大小？ #

First off, let's parse what this declaration actually means: std::string* str_arr_p[] defines an array of pointers to std::string objects (often called a "pointer array").

Whether you need to specify the size depends on where you declare it:

• Global/namespace scope or static class members: You can omit the size here. The compiler will let you declare it without a size, as long as you provide an initializer later (either in the same translation unit or via extern). For example:

  // Global scope declaration (no size specified)
  std::string* str_arr_p[];
  
  // Later in the same file, define and initialize it
  std::string s1 = "foo", s2 = "bar";
  std::string* str_arr_p[] = {&s1, &s2}; // Compiler infers size as 2
  

• Local scope (e.g., inside a function): You must specify the size, or provide an initializer that lets the compiler infer it. If you just write std::string* str_arr_p[]; without a size or initializer in a function, the compiler will throw an error—it can't allocate memory for an array of unknown size on the stack. Example of valid local usage:

  void my_function() {
      std::string a = "hello", b = "world";
      // Valid: size inferred from initializer
      std::string* str_arr_p[] = {&a, &b};
      // Also valid: explicitly specify size
      std::string* str_arr_p2[2] = {&a, &b};
  }
  

2. std::string* str_arr_p[n]; vs std::string (*str_arr_p)[n]; — 核心区别 #

The key here is operator precedence: the [] operator has higher priority than the * operator. Let's break each down:

std::string* str_arr_p[n]; #

This is an array of n pointers to std::string. Each element in the array holds the address of a single std::string object. Think of it as a collection of independent string pointers.

Usage example:

std::string str1 = "apple", str2 = "banana";
std::string* ptr_array[2] = {&str1, &str2};

// Access the first string via the pointer array
std::cout << *ptr_array[0] << std::endl; // Outputs "apple"

std::string (*str_arr_p)[n]; #

This is a pointer to an array of n std::string objects (often called an "array pointer"). The variable itself is a single pointer that points to the entire array of strings, not individual pointers.

Usage example:

std::string str_array[2] = {"cat", "dog"};
std::string (*array_ptr)[2] = &str_array;

// Access the first string via the array pointer
// Note: We need to dereference the pointer first to get the array, then index into it
std::cout << (*array_ptr)[0] << std::endl; // Outputs "cat"

To sum up:

• std::string* str_arr_p[n]; = Array of n string pointers
• std::string (*str_arr_p)[n]; = Pointer to an array of n strings

内容的提问来源于stack exchange，提问作者Alex Fateev