How to create a temporary variable in C++

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I have a function returning a reference to an instance of my class "record".

record& get_record(int key) {     return lookup(key); } 

That is effective it returns a reference and not by value. Now I modify it a bit.

record& get_record(int key) {     if (valid(key))         return lookup(key);     else {         record x;         x.valid=false;         return x; //Here I really want to return a temporary variable                   // and not a reference to a local variable.           } } 

Is it correct that returning a reference to a local variable is not a good idea? and how do I return x in such a way that it becomes a temporary variable?

This is worse than a bad idea, it is undefined behavior and result in most of the cases to a crash. This is bad (TM).

What you could do is changing the return type of get_record so it returns a smart pointer. If key is valid, it returns an observer pointer to it. Otherwise, it returns an owning smart pointer to a freshly created object:

#include <memory> #include <iostream>  struct record { int n; } some_record{42};  std::shared_ptr<record> get_record(bool b) {     if (b == true) {         return std::shared_ptr<record>{&some_record, [](record*){}}; // see explanation ^1     }     return std::shared_ptr<record>{new record{0}}; }  int main() {     std::cout << get_record(true)->n << "/n";  // this is some_record     std::cout << get_record(false)->n << "/n"; // this is a temporary } 

1) About [](record*){}: this no-op lambda given as the second argument to std::shared_ptr::shared_ptr() is invoked when the smart pointer is destroyed. It replaces the default deleter of std::shared_ptr whose behavior is to call delete on the owned pointer.

About why your design is flawed. In fact, making get_record return a reference makes it not consistent. What you want is:

  • if key is valid return a reference to an existing/permanant object, and
  • return a temporary object otherwise.

Those two are mutually exclusive, and your function doesn't make sense: what does get_record return semantically?


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