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Move constructor versus copy constructor behavior on class with no member variables

error: request for member '..' in '..' which is of non-class typeStatic constant string (class member)What is this weird colon-member (“ : ”) syntax in the constructor?Can a class member function template be virtual?Why no default move-assignment/move-constructor?CLI/C++ link error 2028 and 2019Why do we copy then move?Is a `=default` move constructor equivalent to a member-wise move constructor?Copy/move elision versus explicitly deleted copy/move constructorsBehavior of move constructor for string class members

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2

class NullTimer 
 public:
 inline static bool changePeriod (const size_t) return false; 
 inline static void dispose (void) 
 inline static bool isActive (void) return false; 
 inline static void reset (void) 
 inline static void start (void) 
 inline static void stop (void) 
;

template <
 Timer
>
class Foo 
 public:
 Foo (
 const Timer & t
 ) :
 _t(t)
 

 Foo (
 Timer && t
 ) :
 _t(t)
 
 private:
 Timer t;
;

Foo<NullTimer> bar(NullTimer());

Utilizing the move constructor of the templated type is causing bizzare behavior in my embedded application (hard to diagnose because of the embedded nature). If I do it in two steps (utilizing the copy constructor), then it behaves as expected.

NullTimer nt;
Foo<NullTimer> bar(nt);

Can someone explain the fundamental difference(s) in the mechanism of the constructors, and why it would cause different behavior?

c++ templates move-semantics

share|improve this question

asked Mar 27 at 1:08

ZakZak

5,81399 gold badges4141 silver badges8383 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Good ol' Most vexing parse.
  
  – eerorika
  Mar 27 at 1:19
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Most vexing indeed...
  
  – Zak
  Mar 27 at 12:27
  

  
  
  
  

add a comment | 

2

class NullTimer 
 public:
 inline static bool changePeriod (const size_t) return false; 
 inline static void dispose (void) 
 inline static bool isActive (void) return false; 
 inline static void reset (void) 
 inline static void start (void) 
 inline static void stop (void) 
;

template <
 Timer
>
class Foo 
 public:
 Foo (
 const Timer & t
 ) :
 _t(t)
 

 Foo (
 Timer && t
 ) :
 _t(t)
 
 private:
 Timer t;
;

Foo<NullTimer> bar(NullTimer());

Utilizing the move constructor of the templated type is causing bizzare behavior in my embedded application (hard to diagnose because of the embedded nature). If I do it in two steps (utilizing the copy constructor), then it behaves as expected.

NullTimer nt;
Foo<NullTimer> bar(nt);

Can someone explain the fundamental difference(s) in the mechanism of the constructors, and why it would cause different behavior?

c++ templates move-semantics

share|improve this question

asked Mar 27 at 1:08

ZakZak

5,81399 gold badges4141 silver badges8383 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Good ol' Most vexing parse.
  
  – eerorika
  Mar 27 at 1:19
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Most vexing indeed...
  
  – Zak
  Mar 27 at 12:27
  

  
  
  
  

add a comment | 

class NullTimer 
 public:
 inline static bool changePeriod (const size_t) return false; 
 inline static void dispose (void) 
 inline static bool isActive (void) return false; 
 inline static void reset (void) 
 inline static void start (void) 
 inline static void stop (void) 
;

template <
 Timer
>
class Foo 
 public:
 Foo (
 const Timer & t
 ) :
 _t(t)
 

 Foo (
 Timer && t
 ) :
 _t(t)
 
 private:
 Timer t;
;

Foo<NullTimer> bar(NullTimer());

Utilizing the move constructor of the templated type is causing bizzare behavior in my embedded application (hard to diagnose because of the embedded nature). If I do it in two steps (utilizing the copy constructor), then it behaves as expected.

NullTimer nt;
Foo<NullTimer> bar(nt);

Can someone explain the fundamental difference(s) in the mechanism of the constructors, and why it would cause different behavior?

c++ templates move-semantics

share|improve this question

asked Mar 27 at 1:08

ZakZak

5,81399 gold badges4141 silver badges8383 bronze badges

class NullTimer 
 public:
 inline static bool changePeriod (const size_t) return false; 
 inline static void dispose (void) 
 inline static bool isActive (void) return false; 
 inline static void reset (void) 
 inline static void start (void) 
 inline static void stop (void) 
;

template <
 Timer
>
class Foo 
 public:
 Foo (
 const Timer & t
 ) :
 _t(t)
 

 Foo (
 Timer && t
 ) :
 _t(t)
 
 private:
 Timer t;
;

Foo<NullTimer> bar(NullTimer());

NullTimer nt;
Foo<NullTimer> bar(nt);

share|improve this question

asked Mar 27 at 1:08

ZakZak

5,81399 gold badges4141 silver badges8383 bronze badges

share|improve this question

asked Mar 27 at 1:08

ZakZak

5,81399 gold badges4141 silver badges8383 bronze badges

asked Mar 27 at 1:08

ZakZak

5,81399 gold badges4141 silver badges8383 bronze badges

asked Mar 27 at 1:08

ZakZak

5,81399 gold badges4141 silver badges8383 bronze badges

1 Answer
1

active

oldest

votes

3

Foo<NullTimer> bar(NullTimer()); is a function declaration; it declares a function named bar, which returns Foo<NullTimer>, and has a unnamed parameter that is a function pointer returning NullTimer and taking nothing.

You can change it to

// since C++11
Foo<NullTimer> bar(NullTimer);
Foo<NullTimer> barNullTimer();
Foo<NullTimer> barNullTimer;

// before C++11
Foo<NullTimer> bar((NullTimer()));

See Most vexing parse

share|improve this answer

edited Mar 27 at 1:41

answered Mar 27 at 1:12

songyuanyaosongyuanyao

102k1111 gold badges199199 silver badges266266 bronze badges

• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  Foo<NullTimer> bar((NullTimer())) also works, though I think the forms with braces should be preferred for readability reasons.
  
  – Brian
  Mar 27 at 1:20
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @Brian Yes, it's the general workaround before C++11.
  
  – songyuanyao
  Mar 27 at 1:22
  

  
  
  
  

add a comment | 

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3

Foo<NullTimer> bar(NullTimer()); is a function declaration; it declares a function named bar, which returns Foo<NullTimer>, and has a unnamed parameter that is a function pointer returning NullTimer and taking nothing.

You can change it to

// since C++11
Foo<NullTimer> bar(NullTimer);
Foo<NullTimer> barNullTimer();
Foo<NullTimer> barNullTimer;

// before C++11
Foo<NullTimer> bar((NullTimer()));

See Most vexing parse

share|improve this answer

edited Mar 27 at 1:41

answered Mar 27 at 1:12

songyuanyaosongyuanyao

102k1111 gold badges199199 silver badges266266 bronze badges

• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  Foo<NullTimer> bar((NullTimer())) also works, though I think the forms with braces should be preferred for readability reasons.
  
  – Brian
  Mar 27 at 1:20
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @Brian Yes, it's the general workaround before C++11.
  
  – songyuanyao
  Mar 27 at 1:22
  

  
  
  
  

add a comment | 

3

Foo<NullTimer> bar(NullTimer()); is a function declaration; it declares a function named bar, which returns Foo<NullTimer>, and has a unnamed parameter that is a function pointer returning NullTimer and taking nothing.

You can change it to

// since C++11
Foo<NullTimer> bar(NullTimer);
Foo<NullTimer> barNullTimer();
Foo<NullTimer> barNullTimer;

// before C++11
Foo<NullTimer> bar((NullTimer()));

See Most vexing parse

share|improve this answer

edited Mar 27 at 1:41

answered Mar 27 at 1:12

songyuanyaosongyuanyao

102k1111 gold badges199199 silver badges266266 bronze badges

• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  Foo<NullTimer> bar((NullTimer())) also works, though I think the forms with braces should be preferred for readability reasons.
  
  – Brian
  Mar 27 at 1:20
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @Brian Yes, it's the general workaround before C++11.
  
  – songyuanyao
  Mar 27 at 1:22
  

  
  
  
  

add a comment | 

Foo<NullTimer> bar(NullTimer()); is a function declaration; it declares a function named bar, which returns Foo<NullTimer>, and has a unnamed parameter that is a function pointer returning NullTimer and taking nothing.

You can change it to

// since C++11
Foo<NullTimer> bar(NullTimer);
Foo<NullTimer> barNullTimer();
Foo<NullTimer> barNullTimer;

// before C++11
Foo<NullTimer> bar((NullTimer()));

See Most vexing parse

share|improve this answer

edited Mar 27 at 1:41

answered Mar 27 at 1:12

songyuanyaosongyuanyao

102k1111 gold badges199199 silver badges266266 bronze badges

// since C++11
Foo<NullTimer> bar(NullTimer);
Foo<NullTimer> barNullTimer();
Foo<NullTimer> barNullTimer;

// before C++11
Foo<NullTimer> bar((NullTimer()));

share|improve this answer

edited Mar 27 at 1:41

answered Mar 27 at 1:12

songyuanyaosongyuanyao

102k1111 gold badges199199 silver badges266266 bronze badges

answered Mar 27 at 1:12

songyuanyaosongyuanyao

102k1111 gold badges199199 silver badges266266 bronze badges

answered Mar 27 at 1:12

songyuanyaosongyuanyao

102k1111 gold badges199199 silver badges266266 bronze badges