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Questions tagged [stdoptional]

In C++ the std::optional class template represents an optional value, i.e. one that may or may not be present.

The C++ class template std::optional<T> represents an optional value of type T, so that either it contains a valid value of type T or it contains nothing.

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173 questions
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Why does std::optional operator* not have debug mode assertion for has_value()?

I perfectly understand that because of performance reasons the operator* in std::optional does not make any checks for the actual existence of a contained value. However, in debug mode performance considerations should not matter and it would make a…
Francisco Martinez
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Use of std::optional to pass a std::vector to a functional by reference

I am unclear if the correct code to pass an optional vector of ints to a function by reference is : void test_func(std::optional&> vec) or void test_func(std::optional>& vec) Any help much appreciated.
oracle3001
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Does C++ standard allow std::optional to be implemented without overhead

I just watched cppcon talk about Bloomberg datum, variant type that uses redundancy in IEEE754 format to encode what type is stored in datum. So I was wondering does C++ standard allow for implementations to implement std::optional more efficiently…
NoSenseEtAl
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Why std::optional::value() &&; return &&?

I have had runtime error, when replaced some code by using std::optional: Old code: T getValue(); ... const auto& value = getValue(); value.get(); New code: std::optional getValue(); ... const auto& value = getValue().value(); value.get(); //…
Draks
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std::optional::transform with std::addressof

Consider the following code: #include #include template constexpr T * arg_to_pointer(T & arg) noexcept { return std::addressof(arg); } int main() { std::optional foo; auto * test =…
R K
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How can I get clang-tidy to not complain about passing kind-of-lightweight types by value?

CLion (2023.1.2) is running clang-tidy v17 on a project of mine. It's complaining about me passing an std::experimental::optional (from C++14) by value instead of by reference. That seems egregious to me... how can I increase the "object weight…
einpoklum
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prevent initializing std::optional> with rvalue std::optional

std::reference_wrapper cannot be bound to rvalue reference to prevent dangling pointer. However, with combination of std::optional, it seems that rvalue could be bound. That is, std::is_constructible_v, int&&>) is…
slyx
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Does std::optional<>::emplace() invalidate references to the inner value?

Consider the following fragment (assume that T is trivially constructible and trivially destructible): std::optional opt; opt.emplace(); T& ref = opt.value(); opt.emplace(); // is ref guaranteed to be valid here? From the definition of…
Pavel Kirienko
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What are the advantages/disadvantages of std::optional over nullptr?

I looked several online std::optional documentary over the internet. However I could not be able to find any direct comparison between two cases below: case 1: SomePointer* foo::get_some_pointer(cont int value) { auto result =…
Meric Ozcan
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Are std::optional members stored contiguously?

I suppose I'm a bit confused as to how exactly optional values are stored. When constructing a class or struct that contains std::optional members, will these members be stored contiguously in memory or does optional allocate dynamically? For…
whitwhoa
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How to use second overload of std::optional::emplace

In the std::optional::emplace docs there is an overload that accepts std::initializer_list: template< class U, class... Args > T& emplace( std::initializer_list ilist, Args&&... args ); provided that std::is_constructible
pptaszni
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Why is `std::optional::operator=` deleted when T contains a `const` data member?

The following code would lead to compiler errors: #include class A { }; class B { private: const A a; }; int main() { B b; std::optional bo1; bo1 = b; } On gcc, for example, the error reads: main.cpp: In function…
DXZ
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How to get std::optional support in Xcode?

I'm trying to use std::optional in an Xcode 12.0 Mac OS project. I'm getting the error: No template named 'optional' in namespace 'std' #include std::optional o; My settings are (I need libc++ for project):
bhartsb
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Can I somehow return std::optional with const-reference to my data to avoid copy of it?

Suppose I have some base class which can optionally return me some specific data. It also provides me 'hasData' function to check if such specific data is available for usage class MyClassBase { public: virtual bool hasData() const { return…
Michael Nosov
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Why std::string append is not overloaded on rval ref?

I'm familiar, that the append in std::string returns std::string& and therefore it do not qualify to be moved from, so the result will not be moved. #include int main() { std::string s = std::string("A").append("B"); return…
koscelansky
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