std::string to C string

• 使用 std::basic_string::c_str 方法將字串轉換為 char 陣列

• 使用 std::vector 容器將字串轉換為 Char 陣列
• 使用指標操作操作將字串轉換為字元陣列

c_str()

• return const char*

  string tmp_string = "This will be converted to char*";
  auto c_string = tmp_string.c_str();

string.data()

• A pointer to the c-string representation of the string object’s value.
• The pointer returned may be invalidated by further calls to other member functions that modify the object.
• Returns a pointer to an array that contains a null-terminated sequence of characters (i.e., a C-string) representing the current value of the string object.
  • This array includes the same sequence of characters that make up the value of the string object plus an additional terminating null-character (‘\0’) at the end.

    string str = "some string" ;
    char *cstr = str.data();

由 const char* 轉成 char*

1. 直接修改函式參數的型態定義，但原本函式庫裡的宣告根本不能改。
2. 用 const_cast<char*>(cptr)，這個雖然可以強制轉換，但若透過轉換後的指標更改常數的值，將會是 undefined behavior。
3. 使用上一篇提到的 strcpy()，但小心緩衝區覆蓋，或是使用到不知道指到什麼的指標。
4. Declare C a const, i.e. const char *C = …
   • const char *C = R.c_str();
5. Copy the content into space that you have allocated.

   char *C = new char[R.size()+1];
   std::strcpy(C, R.c_str());
   // The second problem is a memory leak: your code assigns C a result of new, but never deletes it. If you use strcpy approach, you need to add
   
   delete[] C;

strncpy

• cstring

• char* strncpy(char* destination, const char* source, size_t num);
• Copies the first num characters of source to destination.
  • If the end of the source C string (which is signaled by a null-character) is found before num characters have been copied, destination is padded with zeros until a total of num characters have been written to it.

  • No null-character is implicitly appended at the end of destination if source is longer than num.

    • Thus, in this case, destination shall not be considered a null terminated C string (reading it as such would overflow).

      char str1[]= "To be or not to be";
      char str2[40];
      char str3[40];
      
      /* copy to sized buffer (overflow safe): */
      strncpy ( str2, str1, sizeof(str2) );
      
      /* partial copy (only 5 chars): */
      strncpy ( str3, str2, 5 );
      str3[5] = '\0';   /* null character manually added */

    vector

• std::vector<char> cstr(str.c_str(), str.c_str() + str.size() + 1);

  reference

• 如何在 C++ 中把字串轉換為 Char 陣列 | D棧 - Delft Stack
• [C] 每天來點字串用法 (3) － from const char* to char*
• c++ - error: invalid conversion from ‘const char*’ to ‘char*’ [-fpermissive] - Stack Overflow
• strncpy - C++ Reference
• c++ - std::string to char* - Stack Overflow
• string::data - C++ Reference