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Re: casting from 'const string' to a 'non-constant string'

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On 10/19/2012 8:36 AM, Rene Ivon Shamberger wrote:
> const std::string& someClass::someMethod(){ return some_string = "Bla Bla Bla"; }

What's 'some_string'?  A member?  A global variable?

> ....
> const std::string& myMethod(){
>   someClass obj;
>   return obj.someMethod();

Right after this expression is evaluated and its value is prepared to be 
returned, the 'obj' object is destroyed.  *If* 'someMethod' returns part 
of the object for which it's called, as a reference to const, that 
reference becomes *invalid* as soon as 'obj' is destroyed, i.e. outside 
of the 'myMethod' function.  IOW, you can't use the return value of the 
'myMethod' function at all - that's undefined behavior.

> }
>
> Left as it is, this example will give me a warning stating that the return value from myMethod is a local value, but if I change the code to:
>
> const std::string&
> someClass::someMethod(){ return some_string = "Bla Bla Bla"; }
> const std::string& myMethod(){
>   someClass obj;
>   std::string tmp = obj.someMethod(); /// New code
>   return temp;
> }
> The compiler complains saying that conversion from 'const string' to 'string' is not permited.

On which line?  What's 'temp'?  Is it a global variable?  Where is this 
'myMethod' function defined?  A namespace scope or inside another class?

> How can I remove this error?

Don't "remove this error".  Code correctly.  Do not return references to 
local objects.  Ever.

Also, read the FAQ 5.8.

V
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Thread

casting from 'const string' to a 'non-constant string' Rene Ivon Shamberger <arbolone@gmail.com> - 2012-10-19 05:36 -0700
  Re: casting from 'const string' to a 'non-constant string' Juha Nieminen <nospam@thanks.invalid> - 2012-10-19 12:55 +0000
  Re: casting from 'const string' to a 'non-constant string' Victor Bazarov <v.bazarov@comcast.invalid> - 2012-10-19 14:42 -0400
  Re: casting from 'const string' to a 'non-constant string' Andy Champ <no.way@nospam.invalid> - 2012-10-19 21:43 +0100

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