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Why is
void foo(T*);
illegal in both C and C++ (because T is undefined), whereas
void foo(struct T*);
is valid, even though it still does not define T? Is there any situation in which it makes a semantic difference to the caller whether T is a struct or some other type (class/enum/typedef/etc.)?
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That means a pointer initialized as an integer can only store an address of integer datatype. So when used for pointing(storing address) another datatype than initialized datatype illegal pointer error can occur.
Similarly, we can have a Pointer to Structures, In which the pointer variable point to the address of the user-defined data types i.e. Structures. Structure in C is a user-defined data type which is used to store heterogeneous data in a contiguous manner.
Pointer to structure holds the add of the entire structure. It is used to create complex data structures such as linked lists, trees, graphs and so on. The members of the structure can be accessed using a special operator called as an arrow operator ( -> ).
Structure pointer points to the address of the structure variable in the memory block to which it points. This pointer can be used to access and change the value of structure members. This way, structures and pointers in C can be used to conveniently create and access user-defined data types.
void foo(struct T*); simultaneously functions as a forward-declaration of struct T within that scope, so it's safe to name a pointer to it.
In void foo(T*) there is no telling what T is supposed to be. If it turns out to be the name of a variable rather than a type, then the line of code is ill-formed. C++ needs more information before it can say that the line of code is valid.
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