Kdtyjb

Has any country ever had 2 former presidents in jail simultaneously?

What is this called? Old film camera viewer?

How can "mimic phobia" be cured or prevented?

How to implement a feedback to keep the DC gain at zero for this conceptual passive filter?

Intuition of generalized eigenvector.

If a character has darkvision, can they see through an area of nonmagical darkness filled with lightly obscuring gas?

What if a revenant (monster) gains fire resistance?

Biological Blimps: Propulsion

Drawing ramified coverings with tikz

Store Credit Card Information in Password Manager?

Why did the EU agree to delay the Brexit deadline?

On a tidally locked planet, would time be quantized?

How to explain what's wrong with this application of the chain rule?

Create all possible words using a set or letters

"Spoil" vs "Ruin"

Fear of getting stuck on one programming language / technology that is not used in my country

Does a 'pending' US visa application constitute a denial?

Count the occurrence of each unique word in the file

Why electric field inside a cavity of a non-conducting sphere not zero?

Travelling outside the UK without a passport

Is the U.S. Code copyrighted by the Government?

What is the evidence for the "tyranny of the majority problem" in a direct democracy context?

How much character growth crosses the line into breaking the character

Closed-form expression for certain product

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

Why can't I cast a function pointer to (void *)?What are the differences between a pointer variable and a reference variable in C++?Why use static_cast<int>(x) instead of (int)x?When should static_cast, dynamic_cast, const_cast and reinterpret_cast be used?Static constant string (class member)Why does flowing off the end of a non-void function without returning a value not produce a compiler error?Why do we need virtual functions in C++?What does int argc, char *argv[] mean?Why is the new operator allowed to return *void to every pointer-type?Implementation of static_cast operator and it's limitsWhy deleting void* is UB rather than compilation error?

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 4 hours ago

Fabio Turati

2,63452341

asked 4 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
  4 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Potential Dupe: stackoverflow.com/questions/36645660/…
  
  – user4581301
  4 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @user4581301 Not really – the other question is about C, and there might be differences in the languages...
  
  – Aconcagua
  4 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
  3 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 4 hours ago

Fabio Turati

2,63452341

asked 4 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
  4 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Potential Dupe: stackoverflow.com/questions/36645660/…
  
  – user4581301
  4 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @user4581301 Not really – the other question is about C, and there might be differences in the languages...
  
  – Aconcagua
  4 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
  3 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 4 hours ago

Fabio Turati

2,63452341

asked 4 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 4 hours ago

Fabio Turati

2,63452341

asked 4 hours ago

Soulimane MammarSoulimane Mammar

521413

share|improve this question

edited 4 hours ago

Fabio Turati

2,63452341

asked 4 hours ago

Soulimane MammarSoulimane Mammar

521413

edited 4 hours ago

Fabio Turati

2,63452341

edited 4 hours ago

Fabio Turati

2,63452341

asked 4 hours ago

Soulimane MammarSoulimane Mammar

521413

asked 4 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 2 hours ago

answered 4 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
  4 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
  2 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
  2 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
  2 hours ago
  
  
  

  
  
  
  

add a comment | 

Your Answer

StackExchange.ifUsing("editor", function () {
StackExchange.using("externalEditor", function () {
StackExchange.using("snippets", function () {
StackExchange.snippets.init();
});
});
}, "code-snippets");

StackExchange.ready(function() {
var channelOptions = {
tags: "".split(" "),
id: "1"
};
initTagRenderer("".split(" "), "".split(" "), channelOptions);

StackExchange.using("externalEditor", function() {
// Have to fire editor after snippets, if snippets enabled
if (StackExchange.settings.snippets.snippetsEnabled) {
StackExchange.using("snippets", function() {
createEditor();
});
}
else {
createEditor();
}
});

function createEditor() {
StackExchange.prepareEditor({
heartbeatType: 'answer',
autoActivateHeartbeat: false,
convertImagesToLinks: true,
noModals: true,
showLowRepImageUploadWarning: true,
reputationToPostImages: 10,
bindNavPrevention: true,
postfix: "",
imageUploader: {
brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
allowUrls: true
},
onDemand: true,
discardSelector: ".discard-answer"
,immediatelyShowMarkdownHelp:true
});


}
});

draft saved

draft discarded

Sign up or log in

StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});

Sign up using Google

Sign up using Facebook

Sign up using Email and Password

Post as a guest

Name

Email

Required, but never shown

StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fstackoverflow.com%2fquestions%2f55319602%2fwhy-do-compilers-behave-differently-when-static-casting-a-function-to-void%23new-answer', 'question_page');
}
);

Post as a guest

Name

Email

Required, but never shown

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 2 hours ago

answered 4 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
  4 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
  2 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
  2 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
  2 hours 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 2 hours ago

answered 4 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
  4 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
  2 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
  2 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
  2 hours 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 2 hours ago

answered 4 hours ago

eerorikaeerorika

87.2k663134

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

share|improve this answer

edited 2 hours ago

answered 4 hours ago

eerorikaeerorika

87.2k663134

answered 4 hours ago

eerorikaeerorika

87.2k663134

answered 4 hours ago

eerorikaeerorika

87.2k663134