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In computer science, weak typing is a property attributed to the type systems of some programming languages. It is the opposite of strong typing, and consequently the term weak typing has as many different meanings as strong typing does (see strong typing for a list and detailed discussion).

One of the more common definitions states that weakly-typed programming languages are those that support either implicit type conversion (nearly all languages support at least one implicit type conversion), ad-hoc polymorphism (also known as overloading) or both. These less restrictive usage rules can give the impression that strict adherence to typing rules is less important than in strongly-typed languages and hence that the type system is "weaker". However, such languages usually have restrictions on what programmers can do with values of a given type; thus it is possible for a weakly-typed language to be type safe. Moreover, weakly-typed languages may be statically typed, in which case overloading is resolved statically and type conversion operations are inserted by the compiler, or dynamically typed, in which case everything is resolved at run time.

The advantage claimed of weak typing is that it requires less effort on the part of the programmer than strong typing, because the compiler or interpreter implicitly performs certain kinds of conversions. However, one claimed disadvantage is that weakly typed programming systems catch fewer errors at compile time and some of these might still remain after testing has been completed. Two commonly used languages that support many kinds of implicit conversion are C and C++, and it is sometimes claimed that these are weakly typed languages. However, others argue that these languages place enough restrictions on how operands of different types can be mixed, that the two should be regarded as strongly typed languages.

int a = 5;
float b = a;

These languages also allow ignore typedef names for the purposes of type comparison; for example the following is supported, but is disallowed in some languages (which might be said to make these languages be more strongly typed):

typedef int Date;    /* Type to represent a date */
Date a = 12345;
int b = a;       /* What does the coder intend? */

C++ places more restrictions on the handling of enumerated types than C:

enum animal {CAT=0,DOG=2,ANT=3};
enum animal a = CAT;  /* NB The enum is optional in C++ */
enum animal b = 1;    /* This is a warning or error in C++ */