c++ - What are copy elision and return value optimization? -

what copy elision? (named) return value optimization? imply?

in situations can occur? limitations?

• if referenced question, you're looking the introduction.
• for technical overview, see the standard reference.
• see common cases here.

introduction

for technical overview - skip answer.

for common cases copy elision occurs - skip answer.

copy elision optimization implemented compilers prevent (potentially expensive) copies in situations. makes returning value or pass-by-value feasible in practice (restrictions apply).

it's form of optimization elides (ha!) as-if rule - copy elision can applied if copying/moving object has side-effects.

the following example taken wikipedia:

struct c {   c() {}   c(const c&) { std::cout << "a copy made.\n"; } };  c f() {   return c(); }  int main() {   std::cout << "hello world!\n";   c obj = f(); } 

depending on compiler & settings, following outputs are valid:

hello world!
copy made.
copy made.

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hello world!
copy made.

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hello world!

this means fewer objects can created, can't rely on specific number of destructors being called. shouldn't have critical logic inside copy/move-constructors or destructors, can't rely on them being called.

if call copy or move constructor elided, constructor must still exist , must accessible. ensures copy elision not allow copying objects not copyable, e.g. because have private or deleted copy/move constructor.