Shortest undefined behavior sample in C++

Question

What's the shortest, well-formed C++ code that exhibits undefined behavior?

Answer 1

int main(){main;}

3.6.1 Main function [basic.start.main]

3 - [...] The function main shall not be used within a program.

Edit: this is diagnosable, so not UB.

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int main(){for(;;);}

1.10 Multi-threaded executions and data races [intro.multithread]

24 - The implementation may assume that any thread will eventually do one of the following: — terminate, — make a call to a library I/O function, — access or modify a volatile object, or — perform a synchronization operation or an atomic operation.

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int main(){int i=i;}

4.1 Lvalue-to-rvalue conversion [conv.lval]

1 - [...] If the object to which the glvalue refers is [...] uninitialized, a program that necessitates this conversion has undefined behavior.

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//^L.

Here ^L is the form feed character, which is part of the basic character set. 4 characters (a newline is not required per 2.2:2). Undefined behaviour is per

2.8 Comments [lex.comment]

1 - If there is a form-feed or a vertical-tab character in [a //-style] comment, only white-space characters shall appear between it and the new-line that terminates the comment; no diagnostic is required.

Answer 2

\u\
0000

This has eight characters, and has undefined behaviour, according to §2.2/1.

Each instance of a backslash character (\) immediately followed by a new-line character is deleted, splicing physical source lines to form logical source lines. Only the last backslash on any physical source line shall be eligible for being part of such a splice. If, as a result, a character sequence that matches the syntax of a universal-character-name is produced, the behavior is undefined.

Answer 3

#include. /*Imagine a new-line right after the dot*/

§16.2/4:

A preprocessing directive of the form

             #include  pp-tokens   new-line

(that does not match one of the two previous forms) is permitted. [..] If the directive resulting after all replacements does not match one of the two previous forms, the behavior is undefined.

Answer 4

int main(){int i=1>>-1;}

Explanation:

C++98 and C++11 §5.8/1 both state that

The behavior is undefined if the right operand is negative, or greater than or equal to the length in bits of the promoted left operand.

Answer 5

If one is to believe wikipedia, here's a few:

Modifying strings is said to cause undefined behavior. It's always worked for me.

int main(int c,char*v){v[0]='.';}

A non-void function with no return causes undefined return values.

int a(){}
int main(){return a();}

Division (of int?) by zero is supposedly undefined. All I know is that it crashes.

int main(int c){c/0;}

Answer 6

int main() { int* a; return a[0]; }