error C2440: '=' : cannot convert from 'std::string' to 'char *'

[zhshqzyc]

I am going to form arrays through string. How to convert from std:string to char.

#include<iostream>
#include<string>
#include<fstream>
#include<sstream>
 
	using namespace std;
int main()
{
char *str1[5], *str2[5], *test_string[5];
int  *input, i;
ifstream in("testdfa.txt");  //open input file
string inbuf;
string old_state, symbol, new_state;
 i = 0 ;
while (getline(in, inbuf))  {
	istringstream is (inbuf);
	is >> old_state;
	if (old_state != "FINAL") {
		is >> symbol >> new_state;
		str1[i] = old_state;
		str2[i] = &new_state;
		input[i] = symbol;
		i += 1;
	}
	else
		{test_string[i] = old_string;
	i += 1;}
	}
	cout<<str1[1];
}

c:\documents and settings\owner\my documents\visual studio 2005\projects\dfa\dfa\dfa.cpp(20) : error C2440: '=' : cannot convert from 'std::string' to 'char *'
No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called
c:\documents and settings\owner\my documents\visual studio 2005\projects\dfa\dfa\dfa.cpp(21) : error C2440: '=' : cannot convert from 'std::string *__w64 ' to 'char *'
Types pointed to are unrelated; conversion requires reinterpret_cast, C-style cast or function-style cast
c:\documents and settings\owner\my documents\visual studio 2005\projects\dfa\dfa\dfa.cpp(22) : error C2440: '=' : cannot convert from 'std::string' to 'int'
No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called
c:\documents and settings\owner\my documents\visual studio 2005\projects\dfa\dfa\dfa.cpp(26) : error C2065: 'old_string' : undeclared identifier
Build log was saved at "file://c:\Documents and Settings\Owner\My Documents\Visual Studio 2005\Projects\dfa\dfa\Debug\BuildLog.htm"
dfa - 4 error(s), 0 warning(s)
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

[wildfrog]

How to convert from std:string to char.

You can use the std::string.c_str() function:

std::string str = "some string";
const char* psz = str.c_str();

But, the const char* returned from c_str is only valid as long a the std::string object is valid. If you want to keep the char* beyond that you should make a copy:

std::string str = "some string";
char* psz = strdup(str.c_str());

//...

// free string
free(psz);

- petter

[ovidiucucu]

[ Moved thread ]

[spvardhan]

You dont need to do anything just include the foolowing file
#include <sstream> at the begining

Rajavardhan Sarkapally

[NMTop40]

why do you want all these char * arrays? Unless of course you want to pass into a function like execv().

In that case you should use vector<char> instead of string and set up your char* arrays by taking the address of the first element of your vectors. Note that you will need to do a push_back('\0') on each of your vectors.

I actually wrote a wrapper class called ncstring thus:

class ncstring
{
private:
   std::vector< char > m_vec;

public:
   ncstring( const std::string & str )
     : m_vec( str.begin(), str.end() )
   {
     m_vec.push_back( '\0' );
   }

   char * get()
   {
       return &m_vec[0];
   }

   size_t size() const
   {
       return m_vec.size() - 1;
   }
};

I also create a class called argvec which has underlying vector< char * > and effectively builds up what you need for argv/argc arguments. It's fairly straightforward to write and I'll leave it to you, given what you have seen me do above.