Undefined behavior
From cppreference.com
Renders the entire program meaningless if certain rules of the language are violated.
Explanation
The C++ standard precisely defines the observable behavior of every C++ program that does not fall into one of the following classes:
- ill-formed - the program has syntax errors or diagnosable semantic errors.
- A conforming C++ compiler is required to issue a diagnostic, even if it defines a language extension that assigns meaning to such code (such as with variable-length arrays).
- The text of the standard uses shall, shall not, and ill-formed to indicate these requirements.
- ill-formed, no diagnostic required - the program has semantic errors which may not be diagnosable in general case (e.g. violations of the ODR or other errors that are only detectable at link time).
- The behavior is undefined if such program is executed.
- implementation-defined behavior - the behavior of the program varies between implementations, and the conforming implementation must document the effects of each behavior.
- For example, the type of std::size_t or the number of bits in a byte, or the text of std::bad_alloc::what.
- A subset of implementation-defined behavior is locale-specific behavior, which depends on the implementation-supplied locale.
- unspecified behavior - the behavior of the program varies between implementations, and the conforming implementation is not required to document the effects of each behavior.
- For example, order of evaluation, whether identical string literals are distinct, the amount of array allocation overhead, etc.
- Each unspecified behavior results in one of a set of valid results.
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(since C++26) |
- undefined behavior - there are no restrictions on the behavior of the program.
- Some examples of undefined behavior are data races, memory accesses outside of array bounds, signed integer overflow, null pointer dereference, more than one modifications of the same scalar in an expression without any intermediate sequence point(until C++11)that is unsequenced(since C++11), access to an object through a pointer of a different type, etc.
- Implementations are not required to diagnose undefined behavior (although many simple situations are diagnosed), and the compiled program is not required to do anything meaningful.
UB and optimization
Because correct C++ programs are free of undefined behavior, compilers may produce unexpected results when a program that actually has UB is compiled with optimization enabled:
For example,
Signed overflow
int foo(int x) { return x + 1 > x; // either true or UB due to signed overflow }
may be compiled as (demo)
foo(int): mov eax, 1 ret
Access out of bounds
int table[4] = {}; bool exists_in_table(int v) { // return true in one of the first 4 iterations or UB due to out-of-bounds access for (int i = 0; i <= 4; i++) if (table[i] == v) return true; return false; }
May be compiled as (demo)
exists_in_table(int): mov eax, 1 ret
Uninitialized scalar
std::size_t f(int x) { std::size_t a; if (x) // either x nonzero or UB a = 42; return a; }
May be compiled as (demo)
f(int): mov eax, 42 ret
The output shown was observed on an older version of gcc
Run this code
Possible output:
p is true p is false
Invalid scalar
int f() { bool b = true; unsigned char* p = reinterpret_cast<unsigned char*>(&b); *p = 10; // reading from b is now UB return b == 0; }
May be compiled as (demo)
f(): mov eax, 11 ret
Null pointer dereference
The examples demonstrate reading from the result of dereferencing a null pointer.
int foo(int* p) { int x = *p; if (!p) return x; // Either UB above or this branch is never taken else return 0; } int bar() { int* p = nullptr; return *p; // Unconditional UB }
may be compiled as (demo)
foo(int*): xor eax, eax ret bar(): ret
Access to pointer passed to std::realloc
Choose clang to observe the output shown
Run this code
#include <cstdlib> #include <iostream> int main() { int* p = (int*)std::malloc(sizeof(int)); int* q = (int*)std::realloc(p, sizeof(int)); *p = 1; // UB access to a pointer that was passed to realloc *q = 2; if (p == q) // UB access to a pointer that was passed to realloc std::cout << *p << *q << '\n'; }
Possible output:
12
Infinite loop without side-effects
Choose clang or the latest gcc to observe the output shown.
Run this code
#include <iostream> bool fermat() { const int max_value = 1000; // Non-trivial infinite loop with no side effects is UB for (int a = 1, b = 1, c = 1; true; ) { if (((a * a * a) == ((b * b * b) + (c * c * c)))) return true; // disproved :() a++; if (a > max_value) { a = 1; b++; } if (b > max_value) { b = 1; c++; } if (c > max_value) c = 1; } return false; // not disproved } int main() { std::cout << "Fermat's Last Theorem "; fermat() ? std::cout << "has been disproved!\n" : std::cout << "has not been disproved.\n"; }
Possible output:
Fermat's Last Theorem has been disproved!
Ill-formed with diagnostic message
Note that compilers are permitted to extend the language in ways that give meaning to ill-formed programs. The only thing C++ standard requires in such cases is a diagnostic message (compiler warning), unless the program was "ill-formed no diagnostic required".
For example, unless language extensions are disabled via --pedantic-errors, GCC will compile the following example with only a warning even though it appears in the C++ standard as an example of an "error" (see also GCC Bugzilla #55783)
Run this code
#include <iostream> // Example tweak, do not use constant double a{1.0}; // C++23 standard, §9.4.5 List-initialization [dcl.init.list], Example #6: struct S { // no initializer-list constructors S(int, double, double); // #1 S(); // #2 // ... }; S s1 = {1, 2, 3.0}; // OK, invoke #1 S s2{a, 2, 3}; // error: narrowing S s3{}; // OK, invoke #2 // — end example] S::S(int, double, double) {} S::S() {} int main() { std::cout << "All checks have passed.\n"; }
Possible output:
main.cpp:17:6: error: type 'double' cannot be narrowed to 'int' in initializer ⮠
list [-Wc++11-narrowing]
S s2{a, 2, 3}; // error: narrowing
^
main.cpp:17:6: note: insert an explicit cast to silence this issue
S s2{a, 2, 3}; // error: narrowing
^
static_cast<int>( )
1 error generated.References
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See also
[[assume(expression)]](C++23)
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specifies that the expression will always evaluate to true at a given point (attribute specifier) |
[[indeterminate]](C++26)
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specifies that an object has an indeterminate value if it is not initialized (attribute specifier) |
| (C++23) |
marks unreachable point of execution (function) |
| C documentation for Undefined behavior
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