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Is there sense in using const std::string& arguments in C++17?

How exactly is std::string_view faster than const std::string&?When would I pass const& std::string instead of std::string_view?When do you need a null terminated string in a read-only scenario?Why only string view?What are the differences between a pointer variable and a reference variable in C++?How to convert a std::string to const char* or char*?What is the difference between const int*, const int * const, and int const *?Why is “using namespace std;” considered bad practice?Static constant string (class member)Replacing a 32-bit loop counter with 64-bit introduces crazy performance deviationsenum to string in modern C++11 / C++14 / C++17 and future C++20std::string_view compile time hashingWhat are the new features in C++17?Passing std::string_view to API execting const std::string&

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25

3

By getting string_view in C++17 we got cheap method of passing both std::string and char* to functions that do not take ownership of the string and avoid making temporary copies. By using std::string passed by value and std::move we get explicit and fast passing of string ownership for both r-value and l-value references.

My question is: is there any benefit in using const std::string& as any function parameter in new C++ standard?

c++ c++17

share|improve this question

edited Sep 16 at 7:47

Wai Ha Lee

6,8351313 gold badges4343 silver badges6868 bronze badges

asked Sep 16 at 7:05

bartopbartop

5,7381212 silver badges3939 bronze badges

• 
  
  
  

  
  12
  

  
  
  
  
  
  

  
  
  How exactly is std::string_view faster than const std::string&?, When would I pass const& std::string instead of std::string_view?, Why only string view?, provide various information about that topic. As your question is close to those, you should mention what particular information is missing in those question and their answers.
  
  – t.niese
  Sep 16 at 7:30
  
  

  
  
  
  


• 
  
  
  

  
  4
  

  
  
  
  
  
  

  
  
  In one word: Null-termination.
  
  – Max Langhof
  Sep 16 at 7:37
  

  
  
  
  

add a comment
 | 

25

3

By getting string_view in C++17 we got cheap method of passing both std::string and char* to functions that do not take ownership of the string and avoid making temporary copies. By using std::string passed by value and std::move we get explicit and fast passing of string ownership for both r-value and l-value references.

My question is: is there any benefit in using const std::string& as any function parameter in new C++ standard?

c++ c++17

share|improve this question

edited Sep 16 at 7:47

Wai Ha Lee

6,8351313 gold badges4343 silver badges6868 bronze badges

asked Sep 16 at 7:05

bartopbartop

5,7381212 silver badges3939 bronze badges

• 
  
  
  

  
  12
  

  
  
  
  
  
  

  
  
  How exactly is std::string_view faster than const std::string&?, When would I pass const& std::string instead of std::string_view?, Why only string view?, provide various information about that topic. As your question is close to those, you should mention what particular information is missing in those question and their answers.
  
  – t.niese
  Sep 16 at 7:30
  
  

  
  
  
  


• 
  
  
  

  
  4
  

  
  
  
  
  
  

  
  
  In one word: Null-termination.
  
  – Max Langhof
  Sep 16 at 7:37
  

  
  
  
  

add a comment
 | 

By getting string_view in C++17 we got cheap method of passing both std::string and char* to functions that do not take ownership of the string and avoid making temporary copies. By using std::string passed by value and std::move we get explicit and fast passing of string ownership for both r-value and l-value references.

My question is: is there any benefit in using const std::string& as any function parameter in new C++ standard?

c++ c++17

share|improve this question

edited Sep 16 at 7:47

Wai Ha Lee

6,8351313 gold badges4343 silver badges6868 bronze badges

asked Sep 16 at 7:05

bartopbartop

5,7381212 silver badges3939 bronze badges

share|improve this question

edited Sep 16 at 7:47

Wai Ha Lee

6,8351313 gold badges4343 silver badges6868 bronze badges

asked Sep 16 at 7:05

bartopbartop

5,7381212 silver badges3939 bronze badges

share|improve this question

edited Sep 16 at 7:47

Wai Ha Lee

6,8351313 gold badges4343 silver badges6868 bronze badges

asked Sep 16 at 7:05

bartopbartop

5,7381212 silver badges3939 bronze badges

edited Sep 16 at 7:47

Wai Ha Lee

6,8351313 gold badges4343 silver badges6868 bronze badges

edited Sep 16 at 7:47

Wai Ha Lee

6,8351313 gold badges4343 silver badges6868 bronze badges

asked Sep 16 at 7:05

bartopbartop

5,7381212 silver badges3939 bronze badges

asked Sep 16 at 7:05

bartopbartop

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1 Answer
1

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oldest

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28

Yes.

The problem with std::string_view is that it doesn't remember if it points to a null-terminated string or not.

If you're writing a wrapper for a C api that uses null-terminated strings, you would have constantly copy your std::string_views into std::strings to make sure you have null-terminators.

share|improve this answer

answered Sep 16 at 7:43

HolyBlackCatHolyBlackCat

25.3k44 gold badges4545 silver badges7979 bronze badges

• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  Exactly. Properly written C++17 code should provide overloads for const char *, const std::string & and std::string_view. Especially since many parts of standard library didn't receive string_view support (exceptions, for example).
  
  – StaceyGirl
  Sep 16 at 7:48
  
  

  
  
  
  


• 
  
  
  

  
  17
  

  
  
  
  
  
  

  
  
  @StaceyGirl and here we go again with C++ complexity exploding in the face of the user
  
  – bartop
  Sep 16 at 7:50
  
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  Also, I'm pretty sure std::string_view is slower if one worries so much about micro-performance. Not like it matters, but it really ought to be slower. Passing a string const& will alias the string object which is a pointer in the worst case, and "no op" in the average case. Passing a string_view calls operator string_view to initialize a NRVO-copied string_view which initializes a pointer and a size. When given a const char* rather than a string it will call strlen, so...
  
  – Damon
  Sep 16 at 15:42
  

  
  
  
  


• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  @StaceyGirl Does that mean that many parts of the C++17 standard library, such as exceptions, are not properly written C++17 code?
  
  – wrtlprnft
  Sep 16 at 19:35
  

  
  
  
  


• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  @wrtlprnft Standard exceptions didn't receive string_view constructors and only have const char */const std::string & ones, so if you want to pass a string_view from a derived class, you will have to create an additional string copy - something that string_view was supposed to avoid while reducing number of overloads.
  
  – StaceyGirl
  Sep 16 at 19:40
  

  
  
  
  

add a comment
 | 

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28

Yes.

The problem with std::string_view is that it doesn't remember if it points to a null-terminated string or not.

If you're writing a wrapper for a C api that uses null-terminated strings, you would have constantly copy your std::string_views into std::strings to make sure you have null-terminators.

share|improve this answer

answered Sep 16 at 7:43

HolyBlackCatHolyBlackCat

25.3k44 gold badges4545 silver badges7979 bronze badges

• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  Exactly. Properly written C++17 code should provide overloads for const char *, const std::string & and std::string_view. Especially since many parts of standard library didn't receive string_view support (exceptions, for example).
  
  – StaceyGirl
  Sep 16 at 7:48
  
  

  
  
  
  


• 
  
  
  

  
  17
  

  
  
  
  
  
  

  
  
  @StaceyGirl and here we go again with C++ complexity exploding in the face of the user
  
  – bartop
  Sep 16 at 7:50
  
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  Also, I'm pretty sure std::string_view is slower if one worries so much about micro-performance. Not like it matters, but it really ought to be slower. Passing a string const& will alias the string object which is a pointer in the worst case, and "no op" in the average case. Passing a string_view calls operator string_view to initialize a NRVO-copied string_view which initializes a pointer and a size. When given a const char* rather than a string it will call strlen, so...
  
  – Damon
  Sep 16 at 15:42
  

  
  
  
  


• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  @StaceyGirl Does that mean that many parts of the C++17 standard library, such as exceptions, are not properly written C++17 code?
  
  – wrtlprnft
  Sep 16 at 19:35
  

  
  
  
  


• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  @wrtlprnft Standard exceptions didn't receive string_view constructors and only have const char */const std::string & ones, so if you want to pass a string_view from a derived class, you will have to create an additional string copy - something that string_view was supposed to avoid while reducing number of overloads.
  
  – StaceyGirl
  Sep 16 at 19:40
  

  
  
  
  

add a comment
 | 

28

Yes.

The problem with std::string_view is that it doesn't remember if it points to a null-terminated string or not.

If you're writing a wrapper for a C api that uses null-terminated strings, you would have constantly copy your std::string_views into std::strings to make sure you have null-terminators.

share|improve this answer

answered Sep 16 at 7:43

HolyBlackCatHolyBlackCat

25.3k44 gold badges4545 silver badges7979 bronze badges

• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  Exactly. Properly written C++17 code should provide overloads for const char *, const std::string & and std::string_view. Especially since many parts of standard library didn't receive string_view support (exceptions, for example).
  
  – StaceyGirl
  Sep 16 at 7:48
  
  

  
  
  
  


• 
  
  
  

  
  17
  

  
  
  
  
  
  

  
  
  @StaceyGirl and here we go again with C++ complexity exploding in the face of the user
  
  – bartop
  Sep 16 at 7:50
  
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  Also, I'm pretty sure std::string_view is slower if one worries so much about micro-performance. Not like it matters, but it really ought to be slower. Passing a string const& will alias the string object which is a pointer in the worst case, and "no op" in the average case. Passing a string_view calls operator string_view to initialize a NRVO-copied string_view which initializes a pointer and a size. When given a const char* rather than a string it will call strlen, so...
  
  – Damon
  Sep 16 at 15:42
  

  
  
  
  


• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  @StaceyGirl Does that mean that many parts of the C++17 standard library, such as exceptions, are not properly written C++17 code?
  
  – wrtlprnft
  Sep 16 at 19:35
  

  
  
  
  


• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  @wrtlprnft Standard exceptions didn't receive string_view constructors and only have const char */const std::string & ones, so if you want to pass a string_view from a derived class, you will have to create an additional string copy - something that string_view was supposed to avoid while reducing number of overloads.
  
  – StaceyGirl
  Sep 16 at 19:40
  

  
  
  
  

add a comment
 | 

Yes.

The problem with std::string_view is that it doesn't remember if it points to a null-terminated string or not.

If you're writing a wrapper for a C api that uses null-terminated strings, you would have constantly copy your std::string_views into std::strings to make sure you have null-terminators.

share|improve this answer

answered Sep 16 at 7:43

HolyBlackCatHolyBlackCat

25.3k44 gold badges4545 silver badges7979 bronze badges

share|improve this answer

answered Sep 16 at 7:43

HolyBlackCatHolyBlackCat

25.3k44 gold badges4545 silver badges7979 bronze badges

answered Sep 16 at 7:43

HolyBlackCatHolyBlackCat

25.3k44 gold badges4545 silver badges7979 bronze badges

answered Sep 16 at 7:43

HolyBlackCatHolyBlackCat

25.3k44 gold badges4545 silver badges7979 bronze badges