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