How does a 'const struct' differ from a 'struct'?

What does const struct mean? Is it differ开发者_如何学JAVAent from struct?

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The const part really applies to the variable, not the structure itself.

e.g. @Andreas correctly says:

const struct {
    int x;
    int y;
} foo = {10, 20};
foo.x = 5; //Error

But the important thing is that variable foo is constant, not the struct definition itself. You could equally write that as:

struct apoint {
    int x;
    int y;
};

const struct apoint foo = {10, 20};
foo.x = 5; // Error

struct apoint bar = {10, 20};
bar.x = 5; // Okay

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It means the struct is constant i.e. you can't edit it's fields after it's been initialized.

const struct {
    int x;
    int y;
} foo = {10, 20};
foo.x = 5; //Error

EDIT: GrahamS correctly points out that the constness is a property of the variable, in this case foo, and not the struct definition:

struct Foo {
    int x;
    int y;
};
const struct Foo foo = {10, 20};
foo.x = 5; //Error
struct Foo baz = {10, 20};
baz.x = 5; //Ok

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'const' as the word constant itself indicates means unmodifiable. This can be applied to variable of any data type. struct being a user defined data type, it applies to the the variables of any struct as well. Once initialized, the value of the const variables cannot be modified.

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Const means you cannot edit the field of the structure after the declaration and initialization and you can retrieve the data form the structure

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you can not modify a constant struct ,first struct is a simple data type so when a const key word comes on ,the compiler will held a memory space on a register rather than temporary storage(like ram),and variable identifiers that is stored on register can not be modified