CODEWITHSUNDEEP
cgcctypedef
Felix
Felix

Reputation: 134

Why my attempt to define a natural type with typedef doesn't work?

I tried to define a 'natural' type, like that:

typedef unsigned int nat;

But if I define a nat variable, that variable behaves like an ordinary int:

nat natural_index;
natural_index = 10; // That's what I want. 
natural_index = -10; // Still a valid option.

In resume, I wanted to know why the compiler does not show a message, like "-10 is not a unsigned int", and what could I do to define a 'natural' type.

extra information: I "printf" the variable natural_index, and the value '-10' was printed. I expected at least, another positive number (not exactly 10).

Upvotes: 0

Views: 435

Answers (2)

zwol
zwol

Reputation: 140569

C doesn't support what you are trying to do, on two different levels.

First, a typedef in C does not create a new, distinct type; it just creates a shorthand name for the original type. Thus, after

typedef unsigned int nat;

the declaration

nat natural_index;

is 100% equivalent to

unsigned int natural_index;

(What's the point of typedef, then? It's most useful when the "underlying type" might differ based on the target architecture; for instance, the standard typedef uint64_t might be shorthand for unsigned long or unsigned long long depending on the architecture.)

Second, C has no mechanism for changing whether or not arithmetic expressions perform implicit conversions. In

natural_index = -10;

the assignment operator will convert the negative number -10 (with type int) to a large unsigned number (namely (UINT_MAX - 10) + 1, which is probably, but not necessarily, 4,294,967,286) in the process, and there is no way to disable that.

Your options are to work in a language that actually supports this kind of thing (e.g. Ada, Haskell, ML) or to write a "linting" program that parses C itself and enforces whatever rules you want it to enforce (existing examples are lint and sparse).

Upvotes: 5

alinsoar
alinsoar

Reputation: 15793

you forgot to separate the expression-statements with semicolon ;.

natural_index = 10;
natural_index = -10;

-10 to a large signed number during the evaluation of assignment operator =.

6.5.16.1p2 (Simple assignment) from ISO9899 says:

In simple assignment (=), the value of the right operand is converted to the type of the assignment expression.

When you use the storage class specifier typedef, this tells the parser to add an alias identifier nat for the type unsigned int in the environment of the parser or in the parse tree that it outputs. In this case nat will be evaluated to a type in declarations, so when you declare the object with identifier natural_index, the left-value associated with this object will have the type unsigned int.

Upvotes: 2

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