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c++stlcopyreturn-valuetemporary

Returning a c++ std::vector without a copy?

Is it possible to return a standard container from a function without making a copy?

Example code:

std::vector<A> MyFunc();

...

std::vector<A> b = MyFunc();

As far as I understand, this copies the return value into a new vector b. Does making the function return references or something like that allow avoiding the copy?

Solution

  • If your compiler supports the NRVO then no copy will be made, provided certain conditions are met in the function returning the object. Thankfully, this was finally added in Visual C++ 2005 (v8.0) This can have a major +ve impact on perf if the container is large, obviously.

    If your own compiler docs do not say whether or not it's supported, you should be able to compile the C++ code to assembler (in optimized/release mode) and check what's done using a simple sample function.

    There's also an excellent broader discussion here