C++ - Assigning null to a std::string

user_1546511 0
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I am learning C++ on my own. I have the following code but it gives error. 

#include <iostream>
#include <string>
using namespace std;


int setvalue(const char * value)
{
    string mValue;
    if(value!=0)
    {
       mValue=value;
    }
    else
    {
       mValue=0;
    }
}

int main ()
{
 const char* value = 0;
 setvalue(value);

 cin.get();
 return 0;
}

So want to create a function which accepts char pointers and I want to pass a pointer to it. The function assigns the pointer to its member variable. I'm passing a null pointer intentionally. Following is the error I'm getting: 

 D:\CPP\TestCP.cpp In function `int setvalue(const char*)': 

 note C:\Dev-Cpp\include\c++\3.4.2\bits\basic_string.h:422 candidates are: std::basic_string<_CharT, _Traits, _Alloc>& std::basic_string<_CharT, _Traits, _Alloc>::operator=(const std::basic_string<_CharT, _Traits, _Alloc>&) [with _CharT = char, _Traits = std::char_traits<char>, _Alloc = std::allocator<char>] 

 note C:\Dev-Cpp\include\c++\3.4.2\bits\basic_string.h:422                 std::basic_string<_CharT, _Traits, _Alloc>& std::basic_string<_CharT, _Traits, _Alloc>::operator=(const _CharT*) [with _CharT = char, _Traits = std::char_traits<char>, _Alloc = std::allocator<char>] 

 note C:\Dev-Cpp\include\c++\3.4.2\bits\basic_string.h:422                 std::basic_string<_CharT, _Traits, _Alloc>& std::basic_string<_CharT, _Traits, _Alloc>::operator=(_CharT) [with _CharT = char, _Traits = std::char_traits<char>, _Alloc = std::allocator<char>]

it's basically complaining about line: mValue=0;

Why is it complaining about this line? I can't assign a null to a String?

pointers null c++ string
8 Answers

I won't argue that it's a good idea (or the semantics of using nullptr with things that aren't pointers), but it's relatively simple to create a class which would provide "nullable" semantics (see nullable_string).

However, this is a much better fit for C++17's std::optional:

#include <string>
#include <iostream>
#include <optional>

// optional can be used as the return type of a factory that may fail
std::optional<std::string> create(bool b)
{
    if (b)
        return "Godzilla";
    else
        return {};
}

int main()
{
    std::cout << "create(false) returned "
              << create(false).value_or("empty") << std::endl;

    // optional-returning factory functions are usable as conditions of while and if
    if (auto str = create(true))
    {
        std::cout << "create(true) returned " << *str << std::endl;
    }
}

std::optional, as shown in the example, is convertible to bool, or you may use the has_value() method, has exceptions for bad access, etc. This provides you with nullable semantics, which seems to be what Maria was trying to accomplish.

And if you don't want to wait around for C++17 compatibility, see this answer about Boost.Optional.

user_11363110

compiler gives error because when assigning mValue=0 compiler find assignment operator=(int ) for compile time binding but it's not present in the string class. if we type cast following statement to char like mValue=(char)0 then its compile successfully because string class contain operator=(char) method.

user_106500160
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