I need a MyClass object for the entire lifetime of main(), but only if some flag useMyObject is true.
MyClass myObject runs the constructor. Is that true? I couldn't manage to look it up.The constructor grabs a resource that I only want to grab once and only if the flag is set. This is my code:
int main() { MyClass myObject; // ... if ( useMyObject ) { myObject = MyClass(42); } // ... } An easy way to do is to use std::unique_ptr:
std::unique_ptr<MyClass> myObjectPtr; if(useMyObject) myObjectPtr.reset(new MyClass(42)); if(myObjectPtr) myObjectPtr->method(); But note that it will require a heap allocation, which may be actually more expensive than an "empty" construction. To avoid the cost of heap allocation you can use boost::optional which is used similarly to a pointer, but in fact the storage for the object is allocated on the stack:
boost::optional<MyClass> myObject; if(useMyObject) myObject = MyClass(42); if(myObject) myObject->method(); As you see in any of the above cases, the code will be littered with conditions.
If you are allowed to change the MyClass I would suggest to just construct an "empty" object using the non-parametric constructor. You only need to construct the object into a state where it actually does nothing, but it should not crash the application if used. That empty construction should be cheap and it is only a one time cost anyway.
It would be even better if you can move the object into a scope where it is actually used and avoid the uninitialized/empty object altogether:
if(useMyObject) { MyClass myObject(42); // use the object here and nowhere else. } 
ifs, use &&: myObject && myObject->method(); :P
if, and in there, the object is passed to another class (not developed by me) that uses that object periodically. For some reason, going out of scope after the if would destroy the object.
Yes the constructor will be run when you declare the object.
You could use a std::unique_ptr instead and just initialise the object if required.
However, that will probably leave your code littered with ifs, so there might be a better way to arrange the code.
MyClass& getMyObject() static MyClass myObject(42); return myObject; } Can even be called if you don't actually need the object but just want to ensure that it exists:
int main() { // ... if ( createMyObjectNow ) { getMyObject(); } // ... } 
if. Otherwise, how do you intend to use a non-constructed object?ifends. For example I could have anotherif ( useMyObject )later.MyClass *myObject...myObject = new MyClass(42)?