How to convert a const char * to std::string [duplicate]

Question

What is the correct/best/simplest way to convert a c-style string to a std::string.

The conversion should accept a max_length, and terminate the string at the first \0 char, if this occur before max_length charter.

Answer 1

This page on string::string gives two potential constructors that would do what you want:

string ( const char * s, size_t n ); string ( const string& str, size_t pos, size_t n = npos ); 

Example:

#include<cstdlib> #include<cstring> #include<string> #include<iostream> using namespace std;  int main(){      char* p= (char*)calloc(30, sizeof(char));     strcpy(p, "Hello world");      string s(p, 15);     cout << s.size() << ":[" << s << "]" << endl;     string t(p, 0, 15);     cout << t.size() << ":[" << t << "]" << endl;      free(p);     return 0; } 

Output:

15:[Hello world] 11:[Hello world] 

The first form considers p to be a simple array, and so will create (in our case) a string of length 15, which however prints as a 11-character null-terminated string with cout << .... Probably not what you're looking for.

The second form will implicitly convert the char* to a string, and then keep the maximum between its length and the n you specify. I think this is the simplest solution, in terms of what you have to write.

Answer 2

std::string str(c_str, strnlen(c_str, max_length)); 

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At Christian Rau's request:

strnlen is specified in POSIX.1-2008 and available in GNU's glibc and the Microsoft run-time library. It is not yet found in some other systems; you may fall back to Gnulib's substitute.