Struct 与 type struct

I'm a beginner with C programming, but I was wondering what the difference was between the using typedef when defining a structure versus not using typedef. It seems to my like there's really no difference, they accomplish the same.

struct myStruct{    int one;    int two;};

vs.

typedef struct{    int one;    int two;}myStruct;

The common idiom is using both: typedef struct X { int x; } X;

They are different definitions. To make the discussion clearer I will split the sentence:

struct S { int x; };typedef struct S S;

In the first line you are defining the identifier S within the struct name space (not in the C++ sense). You can use it and define variables or function arguments of the newly defined type by defining the type of the argument as struct S:

void f( struct S argument ); // struct is required here

The second line adds a type alias S in the global name space and thus allows you to just write:

void f( S argument ); // struct keyword no longer needed

Note that since both identifier name spaces are different, defining S both in the structs and global spaces is not an error, as it is not redefining the same identifier, but rather creating a different identifier in a different place.

To make the difference clearer:

typedef struct S { int x; } T;void S() {} // correct//void T() {} // error: symbol T already defined as an alias to 'struct S'

You can define a function with the same name of the struct as the identifiers are kept in different spaces, but you cannot define a function with the same name as a typedef as those identifiers collide.

In C++, it is slightly different as the rules to locate a symbol have changed subtly. C++ still keeps the two different identifier spaces, but unlike in C, when you only define the symbol within the class identifier space, you are not required to provide the struct/class keyword:

 // C++ struct S { int x; }; // S defined as a class void f( S a ); // correct: struct is optional

What changes are the search rules, not where the identifiers are defined. The compiler will search the global identifier table and after S has not been found it will search for S within the class identifiers.

The code presented before behaves in the same way:

typedef struct S { int x; } T;void S() {} // correct [*]//void T() {} // error: symbol T already defined as an alias to 'struct S'

After the definition of the S function in the second line, the struct S cannot be resolved automatically by the compiler, and to create an object or define an argument of that type you must fall back to including the struct keyword:

// previous code here...int main() {    S();     struct S s;}

typedef
typedef type-declaration synonym;
The typedef keyword defines a synonym for the specified type-declaration. The identifier in the type-declaration becomes another name for the type, instead of naming an instance of the type. You cannot use the typedef specifier inside a function definition.
A typedef declaration introduces a name that, within its scope, becomes a synonym for the type given by the decl-specifiers portion of the declaration. In contrast to the class, struct, union, and enum declarations, typedef declarations do not introduce new types — they introduce new names for existing types.
Example
// Example of the typedef keywordtypedef unsigned long ulong;ulong ul;     // Equivalent to "unsigned long ul;"typedef struct mystructtag{   int   i;   float f;   char  c;} mystruct;mystruct ms;   // Equivalent to "struct mystructtag ms;"typedef int (*funcptr)();  // funcptr is synonym for "pointer                           //    to function returning int"funcptr table[10];   // Equivalent to "int (*table[10])();"