extern sample in MSDN
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Hello everyone, I think in practical experience, extern is useful only when we have more than one compile unit (cpp file) and making cross-reference between compile units. In the MSDN extern sample, http://msdn2.microsoft.com/en-us/library/0603949d.aspx It only uses one source file (compile unit) to demonstrate the usage of extern, is it correct and practical? thanks in advance, George
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It only shows the its usage. U'rs thinking is absolutely correct.
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Hi Don, I also think it is correct. But do we achieve special benefits if we use extern in the same compile unit? regards, George
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Hi Don, I also think it is correct. But do we achieve special benefits if we use extern in the same compile unit? regards, George
No. The reason for
externis to differentiate between a declaration and a definition:int num; // Is this a definition or a declaration?In this case the compiler assumes it’s a definition. If you want to sharenumbetween two compilation units this will not do the trick and will result in a linker error – there will be two variables with the same name.extern int num; // This is a declaration.Now this referrers to a “num” defined elsewhere.Steve
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No. The reason for
externis to differentiate between a declaration and a definition:int num; // Is this a definition or a declaration?In this case the compiler assumes it’s a definition. If you want to sharenumbetween two compilation units this will not do the trick and will result in a linker error – there will be two variables with the same name.extern int num; // This is a declaration.Now this referrers to a “num” defined elsewhere.Steve
Thanks Steve, In the sample, even if you do not use why we need to write statement, extern int i? i is global and we can use it anywhere in the same compile unit (cpp file). regards, George
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Thanks Steve, In the sample, even if you do not use why we need to write statement, extern int i? i is global and we can use it anywhere in the same compile unit (cpp file). regards, George
As I said,
externis used to indicate that a construct is a declaration and not a definition. With functions no such mechanism is needed: if the function has a body it’s a definition and if not it’s a declaration:int void FooBar(); // This is a declaration (no body).
int void FooBar() // This is a definition (has body).
{
// Do stuff…
}If you want to use the “FooBar” function in another compilation unit (from where it’s defined) you need to make sure to include its declaration. This is usually done by including a header file. It’s an error to define the same function twice in two separate compilation units however and attempting to do so will result in a link error. Since variables don’t have bodies this technique isn’t usable, thus the
externkeyword:int FooBar; // This is a definition.
extern int FooBar; // This is a declaration.Steve
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As I said,
externis used to indicate that a construct is a declaration and not a definition. With functions no such mechanism is needed: if the function has a body it’s a definition and if not it’s a declaration:int void FooBar(); // This is a declaration (no body).
int void FooBar() // This is a definition (has body).
{
// Do stuff…
}If you want to use the “FooBar” function in another compilation unit (from where it’s defined) you need to make sure to include its declaration. This is usually done by including a header file. It’s an error to define the same function twice in two separate compilation units however and attempting to do so will result in a link error. Since variables don’t have bodies this technique isn’t usable, thus the
externkeyword:int FooBar; // This is a definition.
extern int FooBar; // This is a declaration.Steve
Thanks Steve, How about this, extern int i = 100; a definition or declaration, why? regards, George
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Thanks Steve, How about this, extern int i = 100; a definition or declaration, why? regards, George
From section 3.1.2 of the C++ standard: “A declaration is a definition unless it declares a function without specifying the function’s body (8.4), it contains the extern specifier (7.1.1) or a linkage-specification (7.5) and neither an initializer nor a function-body, it declares a static data member in a class declaration (9.4), it is a class name declaration (9.1), or it is a typedef declaration (7.1.3), a using-declaration (7.3.3), or a using-directive (7.3.4).”
Steve
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From section 3.1.2 of the C++ standard: “A declaration is a definition unless it declares a function without specifying the function’s body (8.4), it contains the extern specifier (7.1.1) or a linkage-specification (7.5) and neither an initializer nor a function-body, it declares a static data member in a class declaration (9.4), it is a class name declaration (9.1), or it is a typedef declaration (7.1.3), a using-declaration (7.3.3), or a using-directive (7.3.4).”
Steve
Thanks Steve, 1. So, extern int i = 100; should be declaration and matches the rule, " it contains the extern specifier (7.1.1)", right? 2. What is " linkage-specification (7.5)"? regards, George
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Hello everyone, I think in practical experience, extern is useful only when we have more than one compile unit (cpp file) and making cross-reference between compile units. In the MSDN extern sample, http://msdn2.microsoft.com/en-us/library/0603949d.aspx It only uses one source file (compile unit) to demonstrate the usage of extern, is it correct and practical? thanks in advance, George
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Thanks Hamid, But it does not cover the case when using extern to qualify a variable which is defined in the same compile unit. :-) regards, George
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Thanks Steve, 1. So, extern int i = 100; should be declaration and matches the rule, " it contains the extern specifier (7.1.1)", right? 2. What is " linkage-specification (7.5)"? regards, George
George_George wrote:
1. So, extern int i = 100; should be declaration and matches the rule, " it contains the extern specifier (7.1.1)", right?
No, because of the "and neither an initializer nor a function-body" clause; the example you gave does have an initializer.
extern int i; // This is a declaration.
extern int i = 100; // This contains an initializer and thus is a definition (and a declaration).George_George wrote:
2. What is " linkage-specification (7.5)"?
extern "C"is an example.Steve
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Thanks Hamid, But it does not cover the case when using extern to qualify a variable which is defined in the same compile unit. :-) regards, George
Doing so is poinless and possibly misleading, but harmless.
Steve
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George_George wrote:
1. So, extern int i = 100; should be declaration and matches the rule, " it contains the extern specifier (7.1.1)", right?
No, because of the "and neither an initializer nor a function-body" clause; the example you gave does have an initializer.
extern int i; // This is a declaration.
extern int i = 100; // This contains an initializer and thus is a definition (and a declaration).George_George wrote:
2. What is " linkage-specification (7.5)"?
extern "C"is an example.Steve
Thanks Stephen, Any special advantage/restrictions/functions we could have when writing extern int i = 3 other than int i = 3? (I think you mean extern int i = 3 has the same meaning as int i = 3, which is definition with initialization. right?) regards, George
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Thanks Stephen, Any special advantage/restrictions/functions we could have when writing extern int i = 3 other than int i = 3? (I think you mean extern int i = 3 has the same meaning as int i = 3, which is definition with initialization. right?) regards, George
There is no reason to do so (use
extern) in such cases. Doing so does neither harm nor good; the two constructs are equivalent. I wouldn’t in cases like that however as it could lead to confusion.Steve
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There is no reason to do so (use
extern) in such cases. Doing so does neither harm nor good; the two constructs are equivalent. I wouldn’t in cases like that however as it could lead to confusion.Steve
Thanks Steve, My question is answered. regards, George