Performance string as function parameter

Question

I have function foo which get std::string as parameter:

void foo(std::string);

and I put a concatence string:

std::string str_1= "example_str_1";
std::string str_2 = "example_str_2";
foo(str_1+str_2);

It is a good way to use a concatence string with std::move?

foo(std::move(str_1+str_2));

Is there any difference between put concatence with std::move?

score 7 · Answer 1 · answered Jan 19 '17 at 12:36

7

No. The purpose of std::move is to cast to an rvalue, but str_1+str_2 is already an rvalue, making the move call redundant.

The larger room for improvement here is in the signature of foo – why does it take a std::string by value? If it only observes the data, the real win here would be changing foo to void foo(std::string const&), or use something akin to C++17's std::string_view.

answered Jan 19 '17 at 12:36

ildjarn

62,044
9
127
211

Thank you for answer, yes of course with foo(std::string const&) – M. S. Jan 19 '17 at 12:48
1

@M.S. : You're welcome. If you need to research value categories further, there's a lot of good information in [this post](http://stackoverflow.com/q/3601602/636019). – ildjarn Jan 19 '17 at 12:49

score 2 · Answer 2 · answered Jan 19 '17 at 12:45

2

There is no difference as std::string has a move constructor.

string (string&& str) noexcept;

Acquires the contents of str.
str is left in an unspecified but valid state.

Ref

answered Jan 19 '17 at 12:45

Rama

3,222
2
11
26

Performance string as function parameter

2 Answers2