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Copy elision for pass-by-value arguments


Copy elision for pass-by-value arguments

By : anthonygallina1
Date : November 21 2020, 07:38 AM
To fix the issue you can do There are actually two copies being performed on each Range object passed to the constructor. The first happens when copying the temporary Range object into the function parameter. This can be elided as per the reference given in 101010's answer. There are specific circumstances in which copy elision can be performed.
The second copy happens when copying the function parameter into the member (as specified in the constructor initialization list). This cannot be elided, and this is why you still see a single copy being made for each parameter in YSC's answer.
code :
Box box(Range(a,b),Range(c,d));
std::cout << box.x.from;
std::cout << a;


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In the absence of copy elision, is the copy/move constructor guaranteed to be called before the destructor?

In the absence of copy elision, is the copy/move constructor guaranteed to be called before the destructor?


By : Stephanux40
Date : March 29 2020, 07:55 AM
seems to work fine Pretty much what the title says. Consider this example: , Yes.
From [stmt.return]/3:
Is copy elision valid in default function arguments?

Is copy elision valid in default function arguments?


By : Eddie S
Date : March 29 2020, 07:55 AM
around this issue As T.C. pointed out, copy-elision may not be possible when passing small trivially-copyable types to functions.
This is because the calling conventions for some ABIs (e.g. the System V ABI) assume that sufficiently small trivially-copyable types will be passed in registers rather than through memory. SysV, for example, will categorise such types in the INTEGER argument class, rather than the MEMORY class.
code :
struct foo {
    foo() { p = this; }
    ~foo(){} //user-provided
};  
Does the behavior of guaranteed copy elision depend on existence of user-defined copy constructor?

Does the behavior of guaranteed copy elision depend on existence of user-defined copy constructor?


By : Prabin Silwal
Date : March 29 2020, 07:55 AM
To fix the issue you can do Quoting from C++17 Working Draft §15.2 Temporary Objects Paragraph 3 (https://timsong-cpp.github.io/cppwp/class.temporary#3):
code :
S(const S &) = default;
S(S &&) = default;
copy initialization : why move or copy constructor was not called even if copy-elision is turned off?

copy initialization : why move or copy constructor was not called even if copy-elision is turned off?


By : user1454181
Date : March 29 2020, 07:55 AM
will help you
why test instance = {1,2} does not call copy constructor or move constructor?
Copy elision of arguments as return values

Copy elision of arguments as return values


By : user3485929
Date : March 29 2020, 07:55 AM
Any of those help
No. We cannot have guaranteed copy elision from a function parameter. If you think about this from an implementation perspective - your argument w has to exist in a register or an address somewhere, so it cannot then be first constructed into the return slot.
code :
Widget frobnicate() {
    Widget w;
    return std::move(w); // oops
}
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