Question

When I am reading some article i found this line is so cryptic.

new (new_ptr + i) T(std::move(old_ptr[i]));

Can somebody explains how this syntax works?

Answer 1

Its a placement.

new (new_ptr + i) T(std::move(old_ptr[i]));

Here we can simplify this to:

new (<destinationLocation>) T(<constructor parameters);

This is normall C++03 and basically allows you to create an object dynamically in an area of memory that you have pre-allocated (its an advanced technique that most people will not use (unless they are building their own container like objects)).

The std::move() part is from C++11 and is creating a special reference type that allowes the move constructor to be used in the type T.

new T(std::move(<source Obj>));

This basically says create a new T using the source object and use the move constructor for effeciency. This means the ‘source Obj’ will be left in an undefined state (thus not usable) after the move but it allows from efficient object creation.

Combining the two you get placement new with move construction using an element from an array as the source object.