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Why isn't it a compile-time error to return a nullptr as a std::string?

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22

1

Due to a bug, I just found out that this code compiles fine on with Visual Studio 17 and probably on other compilers as well. Now I'm curious why?

#include <iostream>
#include <string>

std::string foo()
 return nullptr;


int main()
 auto s = foo();
 std::cout << s << std::endl;

I could imagine it is because the std::basic_string c'tor could be invoked with a char* and while returning an implicit conversion from ptr to std::string occurs (with NULL as argument and then goes poof). Am I on the right way?

c++ function language-lawyer stdstring

share|improve this question

edited Jun 18 at 6:29

Marc.2377

2,43533 gold badges2323 silver badges5757 bronze badges

asked Jun 17 at 11:38

TaronTaron

55055 silver badges2323 bronze badges

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  it is probably indeed passing nullptr as the parameter for a pointer
  
  – Tarick Welling
  Jun 17 at 11:42
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  It compiles, but you get a runtime error.
  
  – t.niese
  Jun 17 at 11:42
  

  
  
  
  


• 
  
  
  

  
  15
  

  
  
  
  
  
  

  
  
  It would probably be beneficial to add a string (nullptr_t) = delete ctor, just so this error is caught at compile time. I don't think legitimate programs would break due to that.
  
  – MSalters
  Jun 17 at 12:03
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Related, Why does constructing std::string(0) not emit a compiler warning?, Access Violation when sending a 0 int literal to a const string parameter, Avoiding improper std::string initialization with NULL const char* using g++, How to best protect from 0 passed to std::string parameters?, etc.
  
  – jww
  Jun 18 at 0:37
  

  
  
  
  

add a comment | 

22

1

Due to a bug, I just found out that this code compiles fine on with Visual Studio 17 and probably on other compilers as well. Now I'm curious why?

#include <iostream>
#include <string>

std::string foo()
 return nullptr;


int main()
 auto s = foo();
 std::cout << s << std::endl;

I could imagine it is because the std::basic_string c'tor could be invoked with a char* and while returning an implicit conversion from ptr to std::string occurs (with NULL as argument and then goes poof). Am I on the right way?

c++ function language-lawyer stdstring

share|improve this question

edited Jun 18 at 6:29

Marc.2377

2,43533 gold badges2323 silver badges5757 bronze badges

asked Jun 17 at 11:38

TaronTaron

55055 silver badges2323 bronze badges

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  it is probably indeed passing nullptr as the parameter for a pointer
  
  – Tarick Welling
  Jun 17 at 11:42
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  It compiles, but you get a runtime error.
  
  – t.niese
  Jun 17 at 11:42
  

  
  
  
  


• 
  
  
  

  
  15
  

  
  
  
  
  
  

  
  
  It would probably be beneficial to add a string (nullptr_t) = delete ctor, just so this error is caught at compile time. I don't think legitimate programs would break due to that.
  
  – MSalters
  Jun 17 at 12:03
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Related, Why does constructing std::string(0) not emit a compiler warning?, Access Violation when sending a 0 int literal to a const string parameter, Avoiding improper std::string initialization with NULL const char* using g++, How to best protect from 0 passed to std::string parameters?, etc.
  
  – jww
  Jun 18 at 0:37
  

  
  
  
  

add a comment | 

Due to a bug, I just found out that this code compiles fine on with Visual Studio 17 and probably on other compilers as well. Now I'm curious why?

#include <iostream>
#include <string>

std::string foo()
 return nullptr;


int main()
 auto s = foo();
 std::cout << s << std::endl;

I could imagine it is because the std::basic_string c'tor could be invoked with a char* and while returning an implicit conversion from ptr to std::string occurs (with NULL as argument and then goes poof). Am I on the right way?

c++ function language-lawyer stdstring

share|improve this question

edited Jun 18 at 6:29

Marc.2377

2,43533 gold badges2323 silver badges5757 bronze badges

asked Jun 17 at 11:38

TaronTaron

55055 silver badges2323 bronze badges

#include <iostream>
#include <string>

std::string foo()
 return nullptr;


int main()
 auto s = foo();
 std::cout << s << std::endl;

share|improve this question

edited Jun 18 at 6:29

Marc.2377

2,43533 gold badges2323 silver badges5757 bronze badges

asked Jun 17 at 11:38

TaronTaron

55055 silver badges2323 bronze badges

share|improve this question

edited Jun 18 at 6:29

Marc.2377

2,43533 gold badges2323 silver badges5757 bronze badges

asked Jun 17 at 11:38

TaronTaron

55055 silver badges2323 bronze badges

edited Jun 18 at 6:29

Marc.2377

2,43533 gold badges2323 silver badges5757 bronze badges

edited Jun 18 at 6:29

Marc.2377

2,43533 gold badges2323 silver badges5757 bronze badges

asked Jun 17 at 11:38

TaronTaron

55055 silver badges2323 bronze badges

asked Jun 17 at 11:38

TaronTaron

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2 Answers
2

active

oldest

votes

24

Yes, your assumption is right, checking std::basic_string constructors #5 will be called:

basic_string( const CharT* s,
 const Allocator& alloc = Allocator() );

Note that passing nullptr invokes undefined behavior as stated in the standard and the notes :

The behavior is
undefined if [s, s + Traits::length(s)) is not a valid range (for
example, if s is a null pointer).

share|improve this answer

edited Jun 17 at 11:53

answered Jun 17 at 11:43

Sombrero ChickenSombrero Chicken

26.5k33 gold badges3535 silver badges8484 bronze badges

add a comment | 

13

Why shouldn't it compile? std::string has the following constructor:

string(const CharT* s, const Allocator& alloc = Allocator());

that constructs the string with the contents initialized with a copy of the null-terminated character string pointed to by s. The constructor is not explicit, so the implicit conversion from nullptr to std::string is indeed possible.

share|improve this answer

edited Jun 17 at 12:16

answered Jun 17 at 11:42

EvgEvg

5,80033 gold badges1919 silver badges4343 bronze badges

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Please note that passing null is UB
  
  – Richard Critten
  Jun 17 at 11:59
  

  
  
  
  


• 
  
  
  

  
  16
  

  
  
  
  
  
  

  
  
  @RichardCritten, it definitely is. But the question was why this code compiles, not what happens if the code is executed. UB will happen not only for nullptr, but for any pointer that doesn't point to the beginning of a null-terminated string.
  
  – Evg
  Jun 17 at 12:13
  

  
  
  
  

add a comment | 

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24

Yes, your assumption is right, checking std::basic_string constructors #5 will be called:

basic_string( const CharT* s,
 const Allocator& alloc = Allocator() );

Note that passing nullptr invokes undefined behavior as stated in the standard and the notes :

The behavior is
undefined if [s, s + Traits::length(s)) is not a valid range (for
example, if s is a null pointer).

share|improve this answer

edited Jun 17 at 11:53

answered Jun 17 at 11:43

Sombrero ChickenSombrero Chicken

26.5k33 gold badges3535 silver badges8484 bronze badges

add a comment | 

24

Yes, your assumption is right, checking std::basic_string constructors #5 will be called:

basic_string( const CharT* s,
 const Allocator& alloc = Allocator() );

Note that passing nullptr invokes undefined behavior as stated in the standard and the notes :

The behavior is
undefined if [s, s + Traits::length(s)) is not a valid range (for
example, if s is a null pointer).

share|improve this answer

edited Jun 17 at 11:53

answered Jun 17 at 11:43

Sombrero ChickenSombrero Chicken

26.5k33 gold badges3535 silver badges8484 bronze badges

add a comment | 

Yes, your assumption is right, checking std::basic_string constructors #5 will be called:

basic_string( const CharT* s,
 const Allocator& alloc = Allocator() );

Note that passing nullptr invokes undefined behavior as stated in the standard and the notes :

The behavior is
undefined if [s, s + Traits::length(s)) is not a valid range (for
example, if s is a null pointer).

share|improve this answer

edited Jun 17 at 11:53

answered Jun 17 at 11:43

Sombrero ChickenSombrero Chicken

26.5k33 gold badges3535 silver badges8484 bronze badges

basic_string( const CharT* s,
 const Allocator& alloc = Allocator() );

share|improve this answer

edited Jun 17 at 11:53

answered Jun 17 at 11:43

Sombrero ChickenSombrero Chicken

26.5k33 gold badges3535 silver badges8484 bronze badges

answered Jun 17 at 11:43

Sombrero ChickenSombrero Chicken

26.5k33 gold badges3535 silver badges8484 bronze badges

answered Jun 17 at 11:43

Sombrero ChickenSombrero Chicken

26.5k33 gold badges3535 silver badges8484 bronze badges

13

Why shouldn't it compile? std::string has the following constructor:

string(const CharT* s, const Allocator& alloc = Allocator());

that constructs the string with the contents initialized with a copy of the null-terminated character string pointed to by s. The constructor is not explicit, so the implicit conversion from nullptr to std::string is indeed possible.

share|improve this answer

edited Jun 17 at 12:16

answered Jun 17 at 11:42

EvgEvg

5,80033 gold badges1919 silver badges4343 bronze badges

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Please note that passing null is UB
  
  – Richard Critten
  Jun 17 at 11:59
  

  
  
  
  


• 
  
  
  

  
  16
  

  
  
  
  
  
  

  
  
  @RichardCritten, it definitely is. But the question was why this code compiles, not what happens if the code is executed. UB will happen not only for nullptr, but for any pointer that doesn't point to the beginning of a null-terminated string.
  
  – Evg
  Jun 17 at 12:13
  

  
  
  
  

add a comment | 

13

Why shouldn't it compile? std::string has the following constructor:

string(const CharT* s, const Allocator& alloc = Allocator());

that constructs the string with the contents initialized with a copy of the null-terminated character string pointed to by s. The constructor is not explicit, so the implicit conversion from nullptr to std::string is indeed possible.

share|improve this answer

edited Jun 17 at 12:16

answered Jun 17 at 11:42

EvgEvg

5,80033 gold badges1919 silver badges4343 bronze badges

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Please note that passing null is UB
  
  – Richard Critten
  Jun 17 at 11:59
  

  
  
  
  


• 
  
  
  

  
  16
  

  
  
  
  
  
  

  
  
  @RichardCritten, it definitely is. But the question was why this code compiles, not what happens if the code is executed. UB will happen not only for nullptr, but for any pointer that doesn't point to the beginning of a null-terminated string.
  
  – Evg
  Jun 17 at 12:13
  

  
  
  
  

add a comment | 

Why shouldn't it compile? std::string has the following constructor:

string(const CharT* s, const Allocator& alloc = Allocator());

that constructs the string with the contents initialized with a copy of the null-terminated character string pointed to by s. The constructor is not explicit, so the implicit conversion from nullptr to std::string is indeed possible.

share|improve this answer

edited Jun 17 at 12:16

answered Jun 17 at 11:42

EvgEvg

5,80033 gold badges1919 silver badges4343 bronze badges

string(const CharT* s, const Allocator& alloc = Allocator());

share|improve this answer

edited Jun 17 at 12:16

answered Jun 17 at 11:42

EvgEvg

5,80033 gold badges1919 silver badges4343 bronze badges

answered Jun 17 at 11:42

EvgEvg

5,80033 gold badges1919 silver badges4343 bronze badges

answered Jun 17 at 11:42

EvgEvg

5,80033 gold badges1919 silver badges4343 bronze badges