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What is going on: C++ std::move on std::shared_ptr increases use_count?

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14

I was always assuming that std::move() on a std::shared_ptr steals the pointer and sets the pointer of the original to nullptr-thus not increasing the reference count. That does not seem to be true in my world.

SETUP:

MacOS, g++ -version => "Apple LLVM version 10.0.1 (clang-1001.0.46.3)"

CODE:

#include <cstdio>                                                                                                                                                                                  

#include <memory>

class Thing { public: Thing(int N) : value(N) {} int value; };



void print(const char* name, std::shared_ptr<Thing>& sp)

{ printf("%s: { use_count=%i; }n", name, (int)sp.use_count()); }



int main(int argc, char** argv) {

    std::shared_ptr<Thing>  x(new Thing(4711));

    print("BEFORE x", x);

    std::shared_ptr<Thing>  y = std::move(x);

    y->value = 4712;

    print(" AFTER x", x);

    print(" AFTER y", y);

    return 0;

}

OUTPUT:

Compiling (g++ tmp.cpp -o test), and running (./test), delivers

BEFORE x: { use_count=1; }

 AFTER x: { use_count=2; }

 AFTER y: { use_count=2; }

So, the reference count is increased while using std::move().

QUESTION:

What is going on, here?

c++ macos shared-ptr c++98

share|improve this question

edited 13 hours ago

Deduplicator

37.4k66 gold badges5252 silver badges9292 bronze badges

asked 15 hours ago

Frank-Rene SchäferFrank-Rene Schäfer

1,37488 silver badges2626 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  After fixing UB can't reproduce.
  
  – Marek R
  14 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  FWIW I'm on (almost) the same version and the library code doesn't match the report iff _LIBCPP_HAS_NO_RVALUE_REFERENCES isn't defined
  
  – Lightness Races in Orbit
  14 hours ago
  
  
  

  
  
  
  

add a comment | 

14

I was always assuming that std::move() on a std::shared_ptr steals the pointer and sets the pointer of the original to nullptr-thus not increasing the reference count. That does not seem to be true in my world.

SETUP:

MacOS, g++ -version => "Apple LLVM version 10.0.1 (clang-1001.0.46.3)"

CODE:

#include <cstdio>                                                                                                                                                                                  

#include <memory>

class Thing { public: Thing(int N) : value(N) {} int value; };



void print(const char* name, std::shared_ptr<Thing>& sp)

{ printf("%s: { use_count=%i; }n", name, (int)sp.use_count()); }



int main(int argc, char** argv) {

    std::shared_ptr<Thing>  x(new Thing(4711));

    print("BEFORE x", x);

    std::shared_ptr<Thing>  y = std::move(x);

    y->value = 4712;

    print(" AFTER x", x);

    print(" AFTER y", y);

    return 0;

}

OUTPUT:

Compiling (g++ tmp.cpp -o test), and running (./test), delivers

BEFORE x: { use_count=1; }

 AFTER x: { use_count=2; }

 AFTER y: { use_count=2; }

So, the reference count is increased while using std::move().

QUESTION:

What is going on, here?

c++ macos shared-ptr c++98

share|improve this question

edited 13 hours ago

Deduplicator

37.4k66 gold badges5252 silver badges9292 bronze badges

asked 15 hours ago

Frank-Rene SchäferFrank-Rene Schäfer

1,37488 silver badges2626 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  After fixing UB can't reproduce.
  
  – Marek R
  14 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  FWIW I'm on (almost) the same version and the library code doesn't match the report iff _LIBCPP_HAS_NO_RVALUE_REFERENCES isn't defined
  
  – Lightness Races in Orbit
  14 hours ago
  
  
  

  
  
  
  

add a comment | 

I was always assuming that std::move() on a std::shared_ptr steals the pointer and sets the pointer of the original to nullptr-thus not increasing the reference count. That does not seem to be true in my world.

SETUP:

MacOS, g++ -version => "Apple LLVM version 10.0.1 (clang-1001.0.46.3)"

CODE:

#include <cstdio>                                                                                                                                                                                  

#include <memory>

class Thing { public: Thing(int N) : value(N) {} int value; };



void print(const char* name, std::shared_ptr<Thing>& sp)

{ printf("%s: { use_count=%i; }n", name, (int)sp.use_count()); }



int main(int argc, char** argv) {

    std::shared_ptr<Thing>  x(new Thing(4711));

    print("BEFORE x", x);

    std::shared_ptr<Thing>  y = std::move(x);

    y->value = 4712;

    print(" AFTER x", x);

    print(" AFTER y", y);

    return 0;

}

OUTPUT:

Compiling (g++ tmp.cpp -o test), and running (./test), delivers

BEFORE x: { use_count=1; }

 AFTER x: { use_count=2; }

 AFTER y: { use_count=2; }

So, the reference count is increased while using std::move().

QUESTION:

What is going on, here?

c++ macos shared-ptr c++98

share|improve this question

edited 13 hours ago

Deduplicator

37.4k66 gold badges5252 silver badges9292 bronze badges

asked 15 hours ago

Frank-Rene SchäferFrank-Rene Schäfer

1,37488 silver badges2626 bronze badges

#include <cstdio>                                                                                                                                                                                  

#include <memory>

class Thing { public: Thing(int N) : value(N) {} int value; };



void print(const char* name, std::shared_ptr<Thing>& sp)

{ printf("%s: { use_count=%i; }n", name, (int)sp.use_count()); }



int main(int argc, char** argv) {

    std::shared_ptr<Thing>  x(new Thing(4711));

    print("BEFORE x", x);

    std::shared_ptr<Thing>  y = std::move(x);

    y->value = 4712;

    print(" AFTER x", x);

    print(" AFTER y", y);

    return 0;

}

BEFORE x: { use_count=1; }

 AFTER x: { use_count=2; }

 AFTER y: { use_count=2; }

share|improve this question

edited 13 hours ago

Deduplicator

37.4k66 gold badges5252 silver badges9292 bronze badges

asked 15 hours ago

Frank-Rene SchäferFrank-Rene Schäfer

1,37488 silver badges2626 bronze badges

share|improve this question

edited 13 hours ago

Deduplicator

37.4k66 gold badges5252 silver badges9292 bronze badges

asked 15 hours ago

Frank-Rene SchäferFrank-Rene Schäfer

1,37488 silver badges2626 bronze badges

edited 13 hours ago

Deduplicator

37.4k66 gold badges5252 silver badges9292 bronze badges

edited 13 hours ago

Deduplicator

37.4k66 gold badges5252 silver badges9292 bronze badges

asked 15 hours ago

Frank-Rene SchäferFrank-Rene Schäfer

1,37488 silver badges2626 bronze badges

asked 15 hours ago

Frank-Rene SchäferFrank-Rene Schäfer

1,37488 silver badges2626 bronze badges

1 Answer
1

active

oldest

votes

30

What is going on, here?

On MacOS, it seems that you must explicitly enable move-sematics with -std=c++11 (or later standards)¹. Otherwise, the example happens to compile (i.e., std::shared_ptr from the related library implementation is usable) but doesn't work correctly as the required language features aren't enabled. This results in actual copies being made instead of move constructions. It would have been better if the AppleClang package didn't even allow an instantiation of std::shared_ptr when the required language features isn't enabled.

¹) Thanks to @t.niese for testing the given compiler/platform.

share|improve this answer

edited 14 hours ago

answered 15 hours ago

lubgrlubgr

25.8k33 gold badges3636 silver badges7979 bronze badges

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @lubgr I can confirm that your code will result on use_count=2; when compiled and run with the given compiler on mac, but will result in use_count=1; on e.g. wandbox.
  
  – t.niese
  14 hours ago
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @lubgr but only when compiling with g++ tmp.cpp -o test, with g++ -std=c++11 tmp.cpp -o test or g++ -std=c++17 tmp.cpp -o test it shows use_count=1;
  
  – t.niese
  14 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  on godbolt it works fine.
  
  – Marek R
  14 hours ago
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  Sounds like the standard library in use allows for using C++11 features (std::shared_ptr) without enabling the language support (move semantics), which results in actual copies being made.
  
  – lubgr
  14 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @lubgr; your last comment seems to pinpoint into the right direction. Can you, or someone, make an answer out of it?
  
  – Frank-Rene Schäfer
  14 hours ago
  

  
  
  
  

 | 
show 3 more comments

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30

What is going on, here?

On MacOS, it seems that you must explicitly enable move-sematics with -std=c++11 (or later standards)¹. Otherwise, the example happens to compile (i.e., std::shared_ptr from the related library implementation is usable) but doesn't work correctly as the required language features aren't enabled. This results in actual copies being made instead of move constructions. It would have been better if the AppleClang package didn't even allow an instantiation of std::shared_ptr when the required language features isn't enabled.

¹) Thanks to @t.niese for testing the given compiler/platform.

share|improve this answer

edited 14 hours ago

answered 15 hours ago

lubgrlubgr

25.8k33 gold badges3636 silver badges7979 bronze badges

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @lubgr I can confirm that your code will result on use_count=2; when compiled and run with the given compiler on mac, but will result in use_count=1; on e.g. wandbox.
  
  – t.niese
  14 hours ago
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @lubgr but only when compiling with g++ tmp.cpp -o test, with g++ -std=c++11 tmp.cpp -o test or g++ -std=c++17 tmp.cpp -o test it shows use_count=1;
  
  – t.niese
  14 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  on godbolt it works fine.
  
  – Marek R
  14 hours ago
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  Sounds like the standard library in use allows for using C++11 features (std::shared_ptr) without enabling the language support (move semantics), which results in actual copies being made.
  
  – lubgr
  14 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @lubgr; your last comment seems to pinpoint into the right direction. Can you, or someone, make an answer out of it?
  
  – Frank-Rene Schäfer
  14 hours ago
  

  
  
  
  

 | 
show 3 more comments

30

What is going on, here?

On MacOS, it seems that you must explicitly enable move-sematics with -std=c++11 (or later standards)¹. Otherwise, the example happens to compile (i.e., std::shared_ptr from the related library implementation is usable) but doesn't work correctly as the required language features aren't enabled. This results in actual copies being made instead of move constructions. It would have been better if the AppleClang package didn't even allow an instantiation of std::shared_ptr when the required language features isn't enabled.

¹) Thanks to @t.niese for testing the given compiler/platform.

share|improve this answer

edited 14 hours ago

answered 15 hours ago

lubgrlubgr

25.8k33 gold badges3636 silver badges7979 bronze badges

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @lubgr I can confirm that your code will result on use_count=2; when compiled and run with the given compiler on mac, but will result in use_count=1; on e.g. wandbox.
  
  – t.niese
  14 hours ago
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @lubgr but only when compiling with g++ tmp.cpp -o test, with g++ -std=c++11 tmp.cpp -o test or g++ -std=c++17 tmp.cpp -o test it shows use_count=1;
  
  – t.niese
  14 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  on godbolt it works fine.
  
  – Marek R
  14 hours ago
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  Sounds like the standard library in use allows for using C++11 features (std::shared_ptr) without enabling the language support (move semantics), which results in actual copies being made.
  
  – lubgr
  14 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @lubgr; your last comment seems to pinpoint into the right direction. Can you, or someone, make an answer out of it?
  
  – Frank-Rene Schäfer
  14 hours ago
  

  
  
  
  

 | 
show 3 more comments

What is going on, here?

On MacOS, it seems that you must explicitly enable move-sematics with -std=c++11 (or later standards)¹. Otherwise, the example happens to compile (i.e., std::shared_ptr from the related library implementation is usable) but doesn't work correctly as the required language features aren't enabled. This results in actual copies being made instead of move constructions. It would have been better if the AppleClang package didn't even allow an instantiation of std::shared_ptr when the required language features isn't enabled.

¹) Thanks to @t.niese for testing the given compiler/platform.

share|improve this answer

edited 14 hours ago

answered 15 hours ago

lubgrlubgr

25.8k33 gold badges3636 silver badges7979 bronze badges

share|improve this answer

edited 14 hours ago

answered 15 hours ago

lubgrlubgr

25.8k33 gold badges3636 silver badges7979 bronze badges

answered 15 hours ago

lubgrlubgr

25.8k33 gold badges3636 silver badges7979 bronze badges

answered 15 hours ago

lubgrlubgr

25.8k33 gold badges3636 silver badges7979 bronze badges