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I'm trying to create a hiearchy of classes in which parts are optional. I would like things to automatically be created as soon as variables are set.

For that I use C++17 std::optional feature.

Now in the example below I forgot to set the "parent" (test2_inst) first, yet g++, clang and msvc all compile and run fine altough with the "not set" output.

My questions now are: am I indeed doing the wrong thing in this example? and what would the proper way of resolving this? Or are the compilers doing the wrong thing?

#include <optional> class test1 { public: class test2 { public: int a, b; class test3 { public: int c, d; }; test3 test3_inst; }; std::optional<test2> test2_inst; }; int main(int argc, char *argv[]) { test1 *test1_inst = new test1(); // can set value test1_inst->test2_inst->test3_inst.c = 3; // yet optional says it is note set? if (test1_inst->test2_inst.has_value()) printf("set\n"); else printf("not set\n"); return 0; }
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    You are using uninitialized variable, which is undefined behavior. That means anything can happen, including correct behavior. Commented Dec 9, 2019 at 10:13
  • @vll which variable is uninitialized? Commented Dec 9, 2019 at 10:22
  • All the memory is stored inside the optional, it's not heap allocated. So you're not going to get a segfault if it's nullopt if the compiler allows it. But the data in there will be incorrect Commented Dec 9, 2019 at 10:29
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    Semi-related: Nothing in C++ is ever guaranteed to crash, so asking "shouldn't this crash?" is always wrong Commented Dec 9, 2019 at 10:38
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    @UnholySheep Off topic: by default, std::terminate calls std::abort which is considered a crash. It is well defined that if you don't catch an exception then std::terminate will be called, so C++ some things are warranted to crash your program. Commented Dec 9, 2019 at 11:00

2 Answers 2

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The behaviour of optional::operator* and optional::operator-> is undefined if the optional does not contain a value.

Accesses the contained value.

  1. Returns a pointer to the contained value.
  2. Returns a reference to the contained value.

The behavior is undefined if *this does not contain a value.

Source: https://en.cppreference.com/w/cpp/utility/optional/operator*

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shouldn't this crash?

Could. Undefined behavior can do anything. Crashing is one possibility. Not crashing and appearing to work is also a possibility.

am I indeed doing the wrong thing in this example?

Yes.

what would the proper way of resolving this?

Depends what you are trying to do. Check the optional...

if (test1_inst->test2_inst) test1_inst->test2_inst->test3_inst.c = 3;

Or, assign its value...

test1_inst->test2_inst = test1::test2{1, 2, {3, 4}};

Or are the compilers doing the wrong thing?

No, the C++ standard gives the compilers a lot of latitude.

C++ is not a nanny language, and gives programmers enough rope to shoot themselves in the foot.

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