convert from string to character array

If you don't want an array with predefined size then you should allocate it dynamically (and remember to delete it when is no longer needed), for instance, assuming str is an instance of std::string, we have:

char * ch1;
ch1 = new char[str.size()+1]; // +1 makes room for string terminator 
strcpy(ch1, str.c_str());
//...
delete [] ch1;

If you would like to do something like

std::string str = !23456789";

// This is invalid...
char ch1[] = str;  

// None of the alternative below are valid.
//char ch1[str.size()] = str;  
//char ch1[] = str.c_str();    
//char ch1[str.size()] = str.c_str();  

In C++, you cannot do something like that. C style array must have a size determined at compile time and there is no way to initialise such an array from a string other than a C-style string defined in code.

Here is a much better alternative if you need to modify the content. If not, then simply use string operator [] or the result of c_str().

std::string str = "123456789";
std::vector<char> ch1(str.begin(), str.end());

// You can know use ch1 almost like an array...