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Why do compilers behave differently when static_cast(ing) a function to void*?

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8

The following code compiles without any error in VSC++2017 and doesn't compile in gcc 7.3.0 (error: invalid static_cast from type ‘int(int)’ to type ‘void*’
void* p = static_cast<void*>(func))

#include <iostream>

int func(int x) return 2 * x; 

int main() 

 void* p = static_cast<void*>(func);
 return 0;

c++ function-pointers void-pointers static-cast

share|improve this question

edited 5 hours ago

Fabio Turati

2,63452341

asked 5 hours ago

Soulimane MammarSoulimane Mammar

521413

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Function pointers are a bit weird. I'd have to go Standard diving, but I'm pretty sure MSVC is bending the Standard for their own nefarious purposes.
  
  – user4581301
  5 hours ago
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @user4581301 Not really – the other question is about C, and there might be differences in the languages...
  
  – Aconcagua
  5 hours ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  While function pointers are different animals than object pointers, most incompatibilities occur when the sizeof() the two differ. If they are the same you can usually safely convert back and forth to a void*. Even so, while it may work, it's not portable and just one of those things best avoided.
  
  – doug
  4 hours ago
  

  
  
  
  

add a comment | 

8

The following code compiles without any error in VSC++2017 and doesn't compile in gcc 7.3.0 (error: invalid static_cast from type ‘int(int)’ to type ‘void*’
void* p = static_cast<void*>(func))

#include <iostream>

int func(int x) return 2 * x; 

int main() 

 void* p = static_cast<void*>(func);
 return 0;

c++ function-pointers void-pointers static-cast

share|improve this question

edited 5 hours ago

Fabio Turati

2,63452341

asked 5 hours ago

Soulimane MammarSoulimane Mammar

521413

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Function pointers are a bit weird. I'd have to go Standard diving, but I'm pretty sure MSVC is bending the Standard for their own nefarious purposes.
  
  – user4581301
  5 hours ago
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @user4581301 Not really – the other question is about C, and there might be differences in the languages...
  
  – Aconcagua
  5 hours ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  While function pointers are different animals than object pointers, most incompatibilities occur when the sizeof() the two differ. If they are the same you can usually safely convert back and forth to a void*. Even so, while it may work, it's not portable and just one of those things best avoided.
  
  – doug
  4 hours ago
  

  
  
  
  

add a comment | 

The following code compiles without any error in VSC++2017 and doesn't compile in gcc 7.3.0 (error: invalid static_cast from type ‘int(int)’ to type ‘void*’
void* p = static_cast<void*>(func))

#include <iostream>

int func(int x) return 2 * x; 

int main() 

 void* p = static_cast<void*>(func);
 return 0;

c++ function-pointers void-pointers static-cast

share|improve this question

edited 5 hours ago

Fabio Turati

2,63452341

asked 5 hours ago

Soulimane MammarSoulimane Mammar

521413

#include <iostream>

int func(int x) return 2 * x; 

int main() 

 void* p = static_cast<void*>(func);
 return 0;

share|improve this question

edited 5 hours ago

Fabio Turati

2,63452341

asked 5 hours ago

Soulimane MammarSoulimane Mammar

521413

share|improve this question

edited 5 hours ago

Fabio Turati

2,63452341

asked 5 hours ago

Soulimane MammarSoulimane Mammar

521413

edited 5 hours ago

Fabio Turati

2,63452341

edited 5 hours ago

Fabio Turati

2,63452341

asked 5 hours ago

Soulimane MammarSoulimane Mammar

521413

asked 5 hours ago

Soulimane MammarSoulimane Mammar

521413

1 Answer
1

active

oldest

votes

5

Functions are implicitly convertible only to function pointers. A function pointer is not a pointer in the strict meaning of the word in the language, which refers only to pointers to objects.

Function pointers cannot be converted to void* using static_cast. The shown program is ill-formed. If a compiler does not warn, then it fails to conform to the standard. Failing to compile an ill-formed program does not violate the standard.

---

On systems where void* is guaranteed to be able to point to a function (such as POSIX), you can use reinterpret_cast instead:

void* p = reinterpret_cast<void*>(func);

But this is not portable to systems that lack the guarantee. (I know of no system that has a C++ compiler and does not have this guarantee, but that does not mean such system does not exist).

Standard quote:

[expr.reinterpret.cast]

Converting a function pointer to an object pointer type or vice versa is conditionally-supported. The meaning
of such a conversion is implementation-defined, except that if an implementation supports conversions in both
directions, converting a prvalue of one type to the other type and back, possibly with different cv-qualification,
shall yield the original pointer value.

Note that this conditional support does not extend to pointers to member functions. Pointers to member functions are not function pointers.

share|improve this answer

edited 3 hours ago

answered 5 hours ago

eerorikaeerorika

87.2k663134

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  POSIX requires interconvertibility between function pointers (code addresses) and object pointers (data addresses), it's necessary to make dlsym work.
  
  – Ben Voigt
  5 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @BenVoigt - that's true, but it won't be done with static_cast. It can be done with reinterpret_cast or a C-style cast.
  
  – Peter
  3 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  On this answer - the standard says nothing about "warnings". The standard requires diagnostics if a program is ill-formed and the error message in the question represents a diagnostic. But the standard neither requires nor forbids warnings (messages about suspicious constructs, etc, that do not prevent code from compiling) - they are discretionary features offered by implementations (and often an indicator of "quality of implementation"). Their absence does not indicate non-compliance with the standard.
  
  – Peter
  3 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Peter the standard requires a diagnostic if the program is ill-formed (unless specified otherwise). The shown program is ill-formed. A diagnostic is required. A warning is a form of a diagnostic, and issuing a warning would be sufficient to conform to the standard. Lack of a diagnostic is violation of the standard.
  
  – eerorika
  3 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @eerorika - You're conflating terms. The term "diagnostic" has a specific definition in the standard as being required by the standard in specified circumstances. Warnings generally provide additional information, not mandated by the standard, because vendors choose to do so, as an aid for developers (usually because the compiler does more analysis than the standard requires, so can provide additional information). To YOU a warning may be a diagnostic. To the C++ standards, by definition in the standards, it is not.
  
  – Peter
  31 mins ago
  

  
  
  
  

add a comment | 

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5

Functions are implicitly convertible only to function pointers. A function pointer is not a pointer in the strict meaning of the word in the language, which refers only to pointers to objects.

Function pointers cannot be converted to void* using static_cast. The shown program is ill-formed. If a compiler does not warn, then it fails to conform to the standard. Failing to compile an ill-formed program does not violate the standard.

---

On systems where void* is guaranteed to be able to point to a function (such as POSIX), you can use reinterpret_cast instead:

void* p = reinterpret_cast<void*>(func);

But this is not portable to systems that lack the guarantee. (I know of no system that has a C++ compiler and does not have this guarantee, but that does not mean such system does not exist).

Standard quote:

[expr.reinterpret.cast]

Converting a function pointer to an object pointer type or vice versa is conditionally-supported. The meaning
of such a conversion is implementation-defined, except that if an implementation supports conversions in both
directions, converting a prvalue of one type to the other type and back, possibly with different cv-qualification,
shall yield the original pointer value.

Note that this conditional support does not extend to pointers to member functions. Pointers to member functions are not function pointers.

share|improve this answer

edited 3 hours ago

answered 5 hours ago

eerorikaeerorika

87.2k663134

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  POSIX requires interconvertibility between function pointers (code addresses) and object pointers (data addresses), it's necessary to make dlsym work.
  
  – Ben Voigt
  5 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @BenVoigt - that's true, but it won't be done with static_cast. It can be done with reinterpret_cast or a C-style cast.
  
  – Peter
  3 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  On this answer - the standard says nothing about "warnings". The standard requires diagnostics if a program is ill-formed and the error message in the question represents a diagnostic. But the standard neither requires nor forbids warnings (messages about suspicious constructs, etc, that do not prevent code from compiling) - they are discretionary features offered by implementations (and often an indicator of "quality of implementation"). Their absence does not indicate non-compliance with the standard.
  
  – Peter
  3 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Peter the standard requires a diagnostic if the program is ill-formed (unless specified otherwise). The shown program is ill-formed. A diagnostic is required. A warning is a form of a diagnostic, and issuing a warning would be sufficient to conform to the standard. Lack of a diagnostic is violation of the standard.
  
  – eerorika
  3 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @eerorika - You're conflating terms. The term "diagnostic" has a specific definition in the standard as being required by the standard in specified circumstances. Warnings generally provide additional information, not mandated by the standard, because vendors choose to do so, as an aid for developers (usually because the compiler does more analysis than the standard requires, so can provide additional information). To YOU a warning may be a diagnostic. To the C++ standards, by definition in the standards, it is not.
  
  – Peter
  31 mins ago
  

  
  
  
  

add a comment | 

5

Functions are implicitly convertible only to function pointers. A function pointer is not a pointer in the strict meaning of the word in the language, which refers only to pointers to objects.

Function pointers cannot be converted to void* using static_cast. The shown program is ill-formed. If a compiler does not warn, then it fails to conform to the standard. Failing to compile an ill-formed program does not violate the standard.

---

On systems where void* is guaranteed to be able to point to a function (such as POSIX), you can use reinterpret_cast instead:

void* p = reinterpret_cast<void*>(func);

But this is not portable to systems that lack the guarantee. (I know of no system that has a C++ compiler and does not have this guarantee, but that does not mean such system does not exist).

Standard quote:

[expr.reinterpret.cast]

Converting a function pointer to an object pointer type or vice versa is conditionally-supported. The meaning
of such a conversion is implementation-defined, except that if an implementation supports conversions in both
directions, converting a prvalue of one type to the other type and back, possibly with different cv-qualification,
shall yield the original pointer value.

Note that this conditional support does not extend to pointers to member functions. Pointers to member functions are not function pointers.

share|improve this answer

edited 3 hours ago

answered 5 hours ago

eerorikaeerorika

87.2k663134

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  POSIX requires interconvertibility between function pointers (code addresses) and object pointers (data addresses), it's necessary to make dlsym work.
  
  – Ben Voigt
  5 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @BenVoigt - that's true, but it won't be done with static_cast. It can be done with reinterpret_cast or a C-style cast.
  
  – Peter
  3 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  On this answer - the standard says nothing about "warnings". The standard requires diagnostics if a program is ill-formed and the error message in the question represents a diagnostic. But the standard neither requires nor forbids warnings (messages about suspicious constructs, etc, that do not prevent code from compiling) - they are discretionary features offered by implementations (and often an indicator of "quality of implementation"). Their absence does not indicate non-compliance with the standard.
  
  – Peter
  3 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Peter the standard requires a diagnostic if the program is ill-formed (unless specified otherwise). The shown program is ill-formed. A diagnostic is required. A warning is a form of a diagnostic, and issuing a warning would be sufficient to conform to the standard. Lack of a diagnostic is violation of the standard.
  
  – eerorika
  3 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @eerorika - You're conflating terms. The term "diagnostic" has a specific definition in the standard as being required by the standard in specified circumstances. Warnings generally provide additional information, not mandated by the standard, because vendors choose to do so, as an aid for developers (usually because the compiler does more analysis than the standard requires, so can provide additional information). To YOU a warning may be a diagnostic. To the C++ standards, by definition in the standards, it is not.
  
  – Peter
  31 mins ago
  

  
  
  
  

add a comment | 

Functions are implicitly convertible only to function pointers. A function pointer is not a pointer in the strict meaning of the word in the language, which refers only to pointers to objects.

Function pointers cannot be converted to void* using static_cast. The shown program is ill-formed. If a compiler does not warn, then it fails to conform to the standard. Failing to compile an ill-formed program does not violate the standard.

---

On systems where void* is guaranteed to be able to point to a function (such as POSIX), you can use reinterpret_cast instead:

void* p = reinterpret_cast<void*>(func);

But this is not portable to systems that lack the guarantee. (I know of no system that has a C++ compiler and does not have this guarantee, but that does not mean such system does not exist).

Standard quote:

[expr.reinterpret.cast]

Converting a function pointer to an object pointer type or vice versa is conditionally-supported. The meaning
of such a conversion is implementation-defined, except that if an implementation supports conversions in both
directions, converting a prvalue of one type to the other type and back, possibly with different cv-qualification,
shall yield the original pointer value.

Note that this conditional support does not extend to pointers to member functions. Pointers to member functions are not function pointers.

share|improve this answer

edited 3 hours ago

answered 5 hours ago

eerorikaeerorika

87.2k663134

void* p = reinterpret_cast<void*>(func);

share|improve this answer

edited 3 hours ago

answered 5 hours ago

eerorikaeerorika

87.2k663134

answered 5 hours ago

eerorikaeerorika

87.2k663134

answered 5 hours ago

eerorikaeerorika

87.2k663134