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

Question

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?

You almost never want to work with char* in C++. For safety and convenience you should use std::string but for a C-style interface you should at least use const char* so you can say str.c_str() but you have to be really careful of object lifetime, for example in your returnString calling str.c_str() will return a dangling pointer to the internal array from the function-local string that fell out of scope. — Cory Kramer
– Cory Kramer, Commented Nov 15, 2021 at 16:50
This is XY problem. Please explain why do you need to return char *? Most probably there is better way to solve your X problem then Y which you have answer for. — Marek R
– Marek R, Commented Nov 15, 2021 at 16:53

Blindy · Accepted Answer · 2021-11-15 16:52:08Z

4

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.

answered Nov 15, 2021 at 16:52

Blindy

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1 Comment

Andreas Wenzel Over a year ago

The function strdup is not part of the ISO C or C++ standard library. It is a platform-specific function (e.g. POSIX). Note that the question is not tagged with a specific platform. Therefore, you may want to recommend using std::strcpy instead, as that function will work on all ISO C++ compliant platforms. However, most common platforms do seem to support strdup, so I am upvoting the answer anyway.

Vlad from Moscow · Accepted Answer · 2021-11-15 17:57:17Z

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 [].

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