Why does this initialization from itself not generate compiler warning?

Question

In this example I have two classes: Test and Test2. Both of their constructors erroneously initialze member variable "val_" from itself, rather than from parameter "val" as intended. The difference is that Test initializes using {} syntax whereas Test2 initializes using () syntax. Only Test2's initialization generates an "initialized with itself" warning.

I'm compiling with -Wall which implies -Winit-from-self. As can be seen from the results printed, both constructors print the wrong value for _val.

909> cat initSelfTest.cc #include <iostream> using namespace std; class Test { public: Test (int val); private: int val_; }; Test::Test (int val) : val_ {val_} { cerr << "Test::Test; val = " << val << "; val_ = " << val_ << "\n"; } class Test2 { public: Test2 (int val); private: int val_; }; Test2::Test2 (int val) : val_ (val_) { cerr << "Test2::Test2; val = " << val << "; val_ = " << val_ << "\n"; } int main (int argc, char **argv) { Test test {781981}; Test2 test2 {781981}; } 910> gcc --version gcc (GCC) 8.2.0 Copyright (C) 2018 Free Software Foundation, Inc. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. 911> g++ -Wall -o initSelfTest initSelfTest.cc initSelfTest.cc: In constructor ‘Test2::Test2(int)’: initSelfTest.cc:23:1: warning: ‘Test2::val_’ is initialized with itself [-Winit-self] Test2::Test2 (int val) ^~~~~ initSelfTest.cc: In constructor ‘Test::Test(int)’: initSelfTest.cc:12:13: warning: ‘*<unknown>.Test::val_’ is used uninitialized in this function [-Wuninitialized] : val_ {val_} ^~~~ initSelfTest.cc: In constructor ‘Test2::Test2(int)’: initSelfTest.cc:24:13: warning: ‘*<unknown>.Test2::val_’ is used uninitialized in this function [-Wuninitialized] : val_ (val_) ^~~~ 912> ./initSelfTest Test::Test; val = 781981; val_ = 0 Test2::Test2; val = 781981; val_ = 0 913>

sidenote: you actually made a good case here for using the same name for the parameter as for the member: Test::Test (int val) : val {val}, no way you can confuse the two ;) — 463035818_is_not_an_ai
– 463035818_is_not_an_ai, Commented Sep 17, 2020 at 14:20

463035818_is_not_an_ai · Accepted Answer · 2020-09-17 14:14:09Z

1

You are using the value before it has been initialized, thats undefined behavior, no error message required/guaranteed. In general warnings are not mandated by the standard.

For the "why is it UB not an error?" I can only speculate. Consider that there are cases that look very similar, but are completely fine. For example

struct foo { foo& f; }; foo f{f};

Only dereferencing would cause a problem, but storing the reference for later use is ok. I can imagine that in general it is impossible for compilers to tell such valid cases apart from the evil ones.

answered Sep 17, 2020 at 14:14

463035818_is_not_an_ai

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5 Comments

David Cogen Over a year ago

But I am using -Wall option to gcc, which implies -Winit-self, which as I understand is specifically intended to catch this. And it does for the () initialization syntax but not for the {} syntax.

463035818_is_not_an_ai Over a year ago

@DavidCogen compiler warnings is an quality of implementation issue. If a compiler feels like it can drown you in warnings or give you no warnings at all. I understood this question to be about C++, if it is about gcc one would need to study the manual or perhaps even dig into implementation details

David Cogen Over a year ago

Ok so maybe I have the wrong forum? Is this not for gcc/g++ questions? If not I'll pursue this elsewhere.

463035818_is_not_an_ai Over a year ago

@DavidCogen no sorry, what I meant is that it is the wrong answer, question is fine. Maybe could be more clear by asking "how do I make gcc generate a warning for this case?"

David Cogen Over a year ago

Ok, I think I'll report as a gcc bug since I'm now convinced that it is.

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