How can I return string as char* from a function in cpp?-CodePudding

for instance,

if str = "ab" is passed in a function returnString()

and we have a function defination like

string returnString(string str) 
{
    str ='c';
    return str;
}

Output :- abc

but if we have function as

char* returnString(string str) 
{
    str ='c';
    return str;
}

will give the error like :

[Error] cannot convert 'std::string' {aka 'std::__cxx11::basic_string<char>'} to 'char*' in return***

How can I resolve this?

CodePudding user response：

You can use str.data() or str.c_str() to get a pointer to the first character, but you should never return this outside of your function. You are inviting a very swift access violation once your string gets destroyed (look carefully, str is a local copy on the function's stack), and that's if you're lucky.

If you're trying to interface with a C API, you can strdup that pointer before exiting the function, but make sure you understand that the new pointer needs to be freed at some later point by you.

CodePudding user response：

There is no implicit conversion from the type std::string to the type char * or const char *.

So the compiler issues the error

[Error] cannot convert 'std::string' {aka 'std::__cxx11::basic_string<char>'} to 'char*' in return

You could declare and define the function the following way.

const char * returnString( std::string &str ) 
{
    str  = 'c';
    return str.c_str();
}

The function changes the passed object of the type std::string. The returned pointer will be valid while the referenced object of the type std::string is alive and is not changed.

Otherwise you need to allocate dynamically a character array. In this case the function could be defined for example like

#include <string>
#include <cstring>

char * returnString( const std::string &str ) 
{
    auto n = str.length();

    char *p = new char[ n   2 ];

    std::memcpy( p, str.c_str(), n );

    p[n] = 'c';
    p[n 1] = '\0';

    return p;
}

You will need to remember to delete the allocated character array when it will not be required any more using the operator delete [].