将 std 字符串转换为 const char*

Question

I am getting an error when I run this piece of code

  string line;
  getline (myfile,line);
  char file_input[15];
  strncpy(file_input, line, 6);
  cout << line << endl;

Error - cannot convert 'std::string' to 'const char*' for argument '2' to 'char* strncpy(char*, const char*, size_t)' How to get rid of this?

Answer 1

Try:

strncpy(file_input, line.c_str(), 6);

This uses the c_str() member function of the std::string type.

Answer 2

strncpy(file_input, line.c_str(), 6);

But why would you want the fixed-length buffer file_input in the first place? It would be safer to construct it as a new std::string containing the first five characters in line :

std::string file_input(line, 0, 5);

Answer 3

Converting from c++ std string to C style string is really easy now.

For that we have string::copy function which will easily convert std string to C style string. reference

string::copy functions parameters serially

1. char string pointer
2. string size, how many characters will b copied
3. position, from where character copy will start

Another important thing,

This function does not append a null character at the end of operation. So, we need to put it manually.

Code exam are in below -

// char string
char chText[20];

// c++ string
string text =  "I am a Programmer";

// conversion from c++ string to char string
// this function does not append a null character at the end of operation
text.copy(chText, text.size(), 0);

// we need to put it manually
chText[text.size()] = '\0';

// below statement prints "I am a Programmer"
cout << chText << endl;

Vice Versa, Converting from C style string to C++ std string is lot more easier

There is three ways we can convert from C style string to C++ std string

First one is using constructor,

char chText[20] = "I am a Programmer";
// using constructor
string text(chText);

Second one is using string::assign method

// char string
char chText[20] = "I am a Programmer";

// c++ string
string text;

// convertion from char string to c++ string
// using assign function
text.assign(chText);

Third one is assignment operator(=), in which string class uses operator overloading

// char string
char chText[20] = "I am a Programmer";

// c++ string
// convertion from char string to c++ string using assignment operator overloading
string text = chText;

third one can be also write like below -

// char string
char chText[20] = "I am a Programmer";

// c++ string
string text;


// convertion from char string to c++ string
text = chText;