Questions tagged [value-categories]
Value categories in C and C++ are a taxonomy of expressions. In C and in C++03, there are lvalues and rvalues. In C++11, there are lvalues, xvalues, and prvalues, as well as the mixed categories glvalues and rvalues.
value-categories
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What is meant by "pr values don't have identity", and why does function call returning an object by value not have identity? [duplicate]
The C++ Standard specifies that a pr-value is (among others) "a function call or an overloaded operator expression, whose return type is non-reference".
Now I wonder why that exactly is the ...
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59
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What is the rationale for the expression class{}.refmem to be an lvalue expression
I came to know that S{}.rval is an lvalue expression. I want to know the rationale behind considering it an lvalue expression.
Why are access to non-static member of reference type lvalue, even when ...
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tool do determine the value category an expression: understanding the result when using references
Looking into definition of value-categories, I tried a snippet proposed here: https://stackoverflow.com/a/16638081/21691539
template <typename T>
struct value_category {
// Or can be an ...
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57
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Why doesn't std::array<T,N>::operator[] return an rvalue reference for rvalue object? [duplicate]
std::array<T,N>::operator[] always returns an lvalue reference (const if needed), but what can possibly be the use of a line of code like this, which is valid C++?
std::array<int,1>{1}[0] =...
- 26.7k
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Making a custom type behave like std::string
I have to define a few structs that contain a member of type std::string. I need to make them behave like std::string. In other words, they should be trivially convertible to std::string but not to ...
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Why ranges::actions::join accepts lvalue ranges unless called with pipe syntax?
I use Range-v3 a lot, but mostly views and algorithms, not very much actions.
As regards ranges::actions::join, why does the following happen?
auto strs{std::vector{"one"s, "...
- 26.7k
1
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3
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Value category of register in C++
I have come across the keyword register in C++ and I am curious about its value category. Are the variables created with the register keyword lvalue (because they can be referenced) or rvalue (because ...
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93
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Is every xvalue an rvalue?
The standard denotes an xvalue as a glvalue whose resources can be reused. It also defines an rvalue as either an xvalue or a prvalue. And a prvalue is defined as an expression that either initializes ...
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Is the rvalue passed to the function still rvalue or not?
I'm trying to understand everything I can about transfer semantics, rvalue, xvalue, prvalue, etc.
I have the following function:
void f(int&& x, int i)
{
std::cout << boost::...
2
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2
answers
128
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How can I concatenate several vectors of unique pointers
The original use-case
The usecase is that I have a function to retrieve some factories derived from FooFactory,
std::vector<std::unique_ptr<FooFactory>> facts = getFactories();
and each ...
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Why argument in std::move looks like (_Ty&& _Arg)? [duplicate]
Why argument in std::move looks like (_Ty&& _Arg)? Why we don't need to write overloads for (_Ty& _Arg)
Why we don't get compilation error, when move l-value reference?
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2
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1
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136
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why different output when run program compiled with gcc and clang
I came across this example below when watching video about c++:
#include <iostream>
#include <type_traits>
struct S {
S() { std::cout << 'a'; }
S(const S&) { std::cout &...
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87
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static_cast to Universal Reference
I'm new to modern c++, and got messed up with value categories.
There is discusssion about 'forward＜T＞ vs static_cast<T&&>'
Which says the purpose of using forward is to write clean code....
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405
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LARAVEL 9: Data from form modals not successfully saved to database when id_jenisizin value == 1 || id_jenisizin == 2
I'm creating an employee leave form, with 3 categories of permissions coming from another table, namely: "Terlambat", "Pulang Cepat ", "Sakit".
select option for ...
1
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1
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380
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Trying to understand: clang's side-effect warnings for typeid on a polymorphic object
This question is not about how to avoid the described warning. (Store in a reference beforehand; Or use dynamic_cast instead of typeid)
I'm trying to understand why the warning exists in the first ...
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