Saturday, July 14, 2012

array-name can't be LHS

An array name myArr looks like a variable but no no no. It is the name plate on a room door.
* It represents an allocated room of fixed size
* It is permanently tied to a fixed address
* You can't put 2 name plates on the same room door
* You can't put this name plate on another door later on.

An array-name in source code is like a const ptr (not ptr-to-const) with a value like 0xFF82, i.e. an alias for Address 0xFF82. Such an Address is not a Lvalue. Therefore array-name isn't a Lvalue. Array-name can't be LHS --

int aa[3];
int b[3];
aa=b; // error

That's trivial, but look at this --

f(aa); // f was declared f(int pa[])

When you pass the array-name "aa" to a function, the array-name is effectively on RHS (not breaking the LHS rule). The LHS is the function parameter-variable "pa", which is always treated as pointer Variable even though you declare the function parameter-variable "pa" as an array-name! Compiler converts it to f(int * pa).

In summary,
* Rule 1 -- array name can't be LHS of an assignment
* Rule 2 -- function param-var is effectively the LHS of an assignment. You can declare a function param-var as array type, which seems to violates the 2 rules above, but
* Rule 2b -- array param-var is converted to pointer param-var

Now let's look at c-string, an important special case.

char const * s = "literal"; // compiler turns a blind eye if you remove the "const" but DON'T

// To modify such a literal string,
char s[] = "literal";
s[0]='A';
cout<<s<<endl;

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