Unpredictable, but valid, behavior of a program that an implementation is not required to document.
Questions tagged [unspecified-behavior]
67 questions
647
votes
9 answers
Undefined, unspecified and implementation-defined behavior
What is undefined behavior (UB) in C and C++? What about unspecified behavior and implementation-defined behavior? What is the difference between them?
Zolomon
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96
votes
2 answers
Does this code from "The C++ Programming Language" 4th edition section 36.3.6 have well-defined behavior?
In Bjarne Stroustrup's The C++ Programming Language 4th edition section 36.3.6 STL-like Operations the following code is used as an example of chaining:
void f2()
{
std::string s = "but I have heard it works even if you don't believe in it" ;
…
Shafik Yaghmour
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72
votes
11 answers
What are the common undefined/unspecified behavior for C that you run into?
An example of unspecified behavior in the C language is the order of evaluation of arguments to a function. It might be left to right or right to left, you just don't know. This would affect how foo(c++, c) or foo(++c, c) gets evaluated.
What other…
Benoit
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54
votes
3 answers
In C99, is f()+g() undefined or merely unspecified?
I used to think that in C99, even if the side-effects of functions f and g interfered, and although the expression f() + g() does not contain a sequence point, f and g would contain some, so the behavior would be unspecified: either f() would be…
Pascal Cuoq
- 79,187
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52
votes
4 answers
Move semantics and function order evaluation
Suppose I have the following:
#include
struct A { int x; };
class B {
B(int x, std::unique_ptr a);
};
class C : public B {
C(std::unique_ptr a) : B(a->x, std::move(a)) {}
};
If I understand the C++ rules about "unspecified…
Matthew Fioravante
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46
votes
1 answer
Can I take the address of a function defined in standard library?
Consider the following code:
#include
#include
#include
int main()
{
std::invoke(std::boolalpha, std::cout); // #1
using ctype_func = int(*)(int);
char c =…
L. F.
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39
votes
4 answers
Is "-1>>5;" unspecified behavior in C?
C11 §6.5.7 Paragraph 5:
The result of E1 >> E2 is E1 right-shifted E2 bit positions. If
E1 has an unsigned type or if E1 has a signed type and a
nonnegative value, the value of the result is the integral part of the
quotient of E1 / 2*^E2. If…
msc
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38
votes
5 answers
How to implement memmove in standard C without an intermediate copy?
From the man page on my system:
void *memmove(void *dst, const void *src, size_t len);
DESCRIPTION
The memmove() function copies len bytes from string src to string dst.
The two strings may overlap; the copy is always done in a…
Pascal Cuoq
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27
votes
3 answers
Is it undefined behaviour if multiple operands in a compound expression modify the same object?
I vaguely remember reading somewhere that it is undefined behaviour if multiple operands in a compound expression modify the same object.
I believe an example of this UB is shown in the code below however I've compiled on g++, clang++ and visual…
ctor
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20
votes
2 answers
Sequence Point ambiguity, undefined behavior?
Today I came across some code that exhibits different behavior on
clang++ (3.7-git), g++ (4.9.2) and Visual Studio 2013. After some reduction
I came up with this snippet which highlights the issue:
#include
using namespace std;
int len_…
Fredrik Axelsson
- 203
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- 5
18
votes
2 answers
Is there a reason why implementations allow instantiation of std::complex with unsupported types
edit note:
originally question said illegal where now it says unspecified.
Thanks to video comment section of Jason Turner video recently I learned that std::complex is unspecified.
But all(AFAIK) implementations seem to happily…
NoSenseEtAl
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18
votes
3 answers
Is it unspecified behavior to compare pointers to different arrays for equality?
The equality operators have the semantic restrictions of relational operators on pointers:
The == (equal to) and the != (not equal to) operators have the same semantic restrictions, conversions, and result type as the relational operators except…
Fred Nurk
- 13,952
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18
votes
5 answers
Strange values while initializing array using designated initializers
When I initialize the array below all the output looks ok except for values[3]. For some reason values[3] initialized as values[0]+values[5] is outputting a very large number. My guess is that I am trying to assign values[0]+values[5] before they…
Luke Murray
- 861
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16
votes
4 answers
Why does a main function without a return statement return value 12?
I have written a program that prints a table. I have not included the return syntax in the main function, but still whenever I type echo $? it displays 12.
My source code :
#include
int main(void)
{
int ans,i,n;
printf("enter the…
zarna
- 263
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15
votes
2 answers
Is there a sequence point between a function call returning an object and a method call on that object?
If I write f(x)->g(args, ...) can I rely on a sequence point after f(x) before the evaluation of args, ...? I can see arguments both ways:
§1.9.17 "When calling a function (whether or not the function is inline), there is a sequence point after…
Ben Jackson
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