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

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14

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++ stdstring

share|improve this question

edited 1 hour ago

Cody Gray♦

198k37391481

asked 13 hours ago

TaronTaron

502520

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  it is probably indeed passing nullptr as the parameter for a pointer
  
  – Tarick Welling
  13 hours ago
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  It compiles, but you get a runtime error.
  
  – t.niese
  13 hours ago
  

  
  
  
  


• 
  
  
  

  
  7
  

  
  
  
  
  
  

  
  
  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
  12 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  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
  17 mins ago
  

  
  
  
  

add a comment | 

14

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++ stdstring

share|improve this question

edited 1 hour ago

Cody Gray♦

198k37391481

asked 13 hours ago

TaronTaron

502520

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  it is probably indeed passing nullptr as the parameter for a pointer
  
  – Tarick Welling
  13 hours ago
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  It compiles, but you get a runtime error.
  
  – t.niese
  13 hours ago
  

  
  
  
  


• 
  
  
  

  
  7
  

  
  
  
  
  
  

  
  
  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
  12 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  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
  17 mins ago
  

  
  
  
  

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++ stdstring

share|improve this question

edited 1 hour ago

Cody Gray♦

198k37391481

asked 13 hours ago

TaronTaron

502520

#include <iostream>
#include <string>

std::string foo()
 return nullptr;


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

share|improve this question

edited 1 hour ago

Cody Gray♦

198k37391481

asked 13 hours ago

TaronTaron

502520

share|improve this question

edited 1 hour ago

Cody Gray♦

198k37391481

asked 13 hours ago

TaronTaron

502520

edited 1 hour ago

Cody Gray♦

198k37391481

edited 1 hour ago

Cody Gray♦

198k37391481

asked 13 hours ago

TaronTaron

502520

asked 13 hours ago

TaronTaron

502520

2 Answers
2

active

oldest

votes

17

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 13 hours ago

answered 13 hours ago

Sombrero ChickenSombrero Chicken

26.3k33484

add a comment | 

8

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 12 hours ago

answered 13 hours ago

EvgEvg

5,56831841

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Please note that passing null is UB
  
  – Richard Critten
  12 hours ago
  

  
  
  
  


• 
  
  
  

  
  7
  

  
  
  
  
  
  

  
  
  @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
  12 hours ago
  

  
  
  
  

add a comment | 

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17

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 13 hours ago

answered 13 hours ago

Sombrero ChickenSombrero Chicken

26.3k33484

add a comment | 

17

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 13 hours ago

answered 13 hours ago

Sombrero ChickenSombrero Chicken

26.3k33484

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 13 hours ago

answered 13 hours ago

Sombrero ChickenSombrero Chicken

26.3k33484

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

share|improve this answer

edited 13 hours ago

answered 13 hours ago

Sombrero ChickenSombrero Chicken

26.3k33484

answered 13 hours ago

Sombrero ChickenSombrero Chicken

26.3k33484

answered 13 hours ago

Sombrero ChickenSombrero Chicken

26.3k33484

8

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 12 hours ago

answered 13 hours ago

EvgEvg

5,56831841

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Please note that passing null is UB
  
  – Richard Critten
  12 hours ago
  

  
  
  
  


• 
  
  
  

  
  7
  

  
  
  
  
  
  

  
  
  @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
  12 hours ago
  

  
  
  
  

add a comment | 

8

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 12 hours ago

answered 13 hours ago

EvgEvg

5,56831841

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Please note that passing null is UB
  
  – Richard Critten
  12 hours ago
  

  
  
  
  


• 
  
  
  

  
  7
  

  
  
  
  
  
  

  
  
  @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
  12 hours ago
  

  
  
  
  

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 12 hours ago

answered 13 hours ago

EvgEvg

5,56831841

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

share|improve this answer

edited 12 hours ago

answered 13 hours ago

EvgEvg

5,56831841

answered 13 hours ago

EvgEvg

5,56831841

answered 13 hours ago

EvgEvg

5,56831841