Quick question, what does the Auto keyword do?

I saw a complete list of C keywords and I didn't know what auto did or some other keywords did.

The ones I can remember that I've never understood fully were :

Code:

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register
auto
...

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Edit : The last one I didn't know was volatile.

register is a request for the compiler to keep that variable available on the (much faster) registers, internally. Pretty much not used anymore, because the compilers are smarter than in decades past. The compiler is free to set up the variables, as it wishes, in any case.

auto is the normal (default) variable, we just don't use it. Inside any function, "int i" for instance, is an auto variable, and has limited scope. Contrasts with static variables, which have an extended scope.

volatile warns the compiler that this variable can have NO assumptions made - typically because another sensor or program, will be changing it, in addition to the current function. Basically, it forces the compiler to re-check it's most current value, before it works with it, even though the function did not change it, since the last time it was given a value.

Auto, register, static, and extern are the four storage classes that C variables can have, with auto being the default. So most variables are implicitly auto. register variables behave just like auto variabes. Register serves as a hint to the compiler to put a variable in a register instead of on the stack. Most modern compilers treat auto and register variables identically.

In C++11 auto has a different meaning.


Volitile is a qualifier that works identially to const. It is used for values that may be modified externally to your program. Unlike in Java, it is not sufficent for multithreaded access.

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Volitile is a qualifier that works identially to const. It is used for values that may be modified externally to your program. Unlike in Java, it is not sufficent for multithreaded access.

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Huh? Volatile and const are quite different. In fact, almost the opposite.

I think King Mir is referring to its use from a syntax point of view.

@Adak: Scope applies to identifiers and storage duration applies to objects. An example:

Code:

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void f() { static int x; }

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'x' would have block scope (and no linkage), and the object associated with 'x' would have static storage duration.

Quote:

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Originally Posted by Barney McGrew

@Adak: Scope applies to identifiers and storage duration applies to objects. An example:

Code:

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void f() { static int x; }

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'x' would have block scope (and no linkage), and the object associated with 'x' would have static storage duration.

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x would actually have "internal" linkage as opposed to no linkage.
Types of linkage:
external: global, non-static vars and funcs
internal : static vars and funcs
none: local vars, typedef names, enum consts

Quote:

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Originally Posted by oogabooga

x would actually have "internal" linkage as opposed to no linkage.
Types of linkage:
external: global, non-static vars and funcs
internal : static vars and funcs
none: local vars, typedef names, enum consts

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x is local, so it has no linkage. You're probably thinking of static variables at file scope.

Quote:

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Originally Posted by cyberfish

Huh? Volatile and const are quite different. In fact, almost the opposite.

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I meant that as a qualifier, it can be used any place const can. As laserlight says, it's got the same syntax.

Quote:

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Originally Posted by laserlight

x is local, so it has no linkage. You're probably thinking of static variables at file scope.

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You're right! I wasn't thinking at all, really. :)

Quote:

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Originally Posted by Barney McGrew

@Adak: Scope applies to identifiers and storage duration applies to objects. An example:

Code:

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void f() { static int x; }

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'x' would have block scope (and no linkage), and the object associated with 'x' would have static storage duration.

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What is "the object associated with 'x'?

Quote:

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Originally Posted by Adak

What is "the object associated with 'x'?

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The "region of data storage in the execution environment" that is associated with the variable named x, the contents of which represents a value as determined by the type of x.

Quote:

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Originally Posted by Adak

What is "the object associated with 'x'?

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It is the entity which, in that definition, is referred to by the name x. The properties of that object include its location in memory, its type (i.e. int), and its value.

An object doesn't necessarily have a name. This particular one does.

Thanks for the definition, grumpy and laserlight.