All Questions
1,031 questions
1
vote
1
answer
27
views
Why is std::conditional reject T&?
I want to return void if template parameter is void, and return T& if template parameter is T. So, I write the next MRE code:
#include<type_traits>
template<class T>
struct Foo {
...
- 952
3
votes
1
answer
137
views
Unclear template resolution
I was going through the exercises in this post https://www.slamecka.cz/posts/2021-03-17-cpp-metaprogramming-exercises-1/
First of all, I want to say huge thanks to the author. The problems were quite ...
- 1,262
4
votes
2
answers
141
views
Idiomatic ways of using tuples and std::tie
I am trying to implement the modular multiplicative inverse using the extended Euclidean algorithm in C++. In Python, the code for this is concise and idiomatic, using tuple unpacking:
def inv_modulo(...
- 1,418
4
votes
3
answers
278
views
Why does std::vector have 2 constructors instead of 1 with default argument?
I looked at cppreference.com and found this
vector();
explicit vector( const Allocator& alloc );
why not just
explicit vector(const Allocator& alloc = Allocator());
1 constructor instead of ...
- 414
0
votes
1
answer
61
views
Is there a way to extract the search from a c++ regex? (not a question about regexs but #include <regex>)
given a C++ std regex is there a way to figure out what that regex would search for? get the string out that you put in when constructing it?
I've seen https://en.cppreference.com/w/cpp/regex/...
- 116
2
votes
1
answer
88
views
How to merge/count adjacent 1's within an std:array using std?
Let say I've an std:array<int> such as:
0 1 1 1 0 1 1 0
I'd like to use std (if possible) to do this in c++11:
sum adjacent 1's
erase the summed value
transform 0 to 1
So getting this result ...
- 47.8k
0
votes
0
answers
38
views
Convert std::function to a function pointer
I have a function from the C library that accepts a function pointer
int c_func(void(*func)(void*), void* data);
I want to write a wrapper for this function that takes a std::function instead of a ...
- 1
0
votes
1
answer
132
views
Why does an std::insert on an std::set of constant-sized arrays not work as expected?
I am in a scenario where I need to process a variable-sized array of constant-sized C-strings, namely: I want to know if each of those strings is unique.
I tried using a std::set, as uniqueness of its ...
- 35
1
vote
1
answer
264
views
How an element of std::map can delete itself from the map?
I have two classes in different source file, class A in a.hpp and a.cpp, class B in b.hpp and b.cpp, A is a Singleton class, it create B object, and store it in a map, when B object iscreated, it ...
- 23
1
vote
1
answer
196
views
Why is the clear() function noexcept while the destructor not? [duplicate]
I noticed that, in the allocator-aware containers interface, the standard requires that the clear() member function be noexcept while the destructor not. Furthermore, it appears a discrepancy in the ...
- 1,794
1
vote
1
answer
84
views
In std::unordered_map, how to iterate over hashes?
The std::unordered_map has hash-value for each key. What is the way to obtain those hash values?
What for? To evaluate relevance of a hash function to the data set. I could just generate hashes from ...
- 11
4
votes
2
answers
155
views
Is there a way to access arguments stored in the function object returned by std::bind() in C++?
I need a way to decompose the function object returned by std::bind() to its arguments in a function template.
The code snippet below shows what I'd like to do:
#include <iostream>
#include <...
5
votes
5
answers
1k
views
Why does std::array require the size as a template parameter and not as a constructor parameter?
There are many design issues I have found with this, particularly with passing std::array<> to functions. Basically, when you initialize std::array, it takes in two template parameters, <...
- 71
1
vote
1
answer
146
views
efficient use of std::move to aggregate all object instances in a std container
I need to centrally accumulate certain entity creations in my program into a container and I want to use the efficiency of std::move with a move constructor to aggregate all entities created anwhere ...
- 530
0
votes
0
answers
29
views
asyncF.cpp:24:31: error: no member named 'async' in namespace 'std'; [duplicate]
I am compiling my code with clang++ and this gives me an error saying
asyncF.cpp:24:31: error: no member named 'async' in namespace 'std'
Here's my code
// c++ program that demonstrates async
// ...
- 23
1
vote
2
answers
961
views
how does std::vector deal with the call to destructor?
When tring to implement std::vector, I get confused with the implicit call to destructor.
Then element in vector maybe:
T
a Class Object,
T*, shared_ptr<T>
a Smart/Simple Pointer to Class ...
- 39
1
vote
2
answers
340
views
To transform std::bind to std::function?
See the code below
queue<function<void()> > tasks;
void add_job(function<void(void*)> func, void* arg) {
function<void()> f = bind(func, arg)();
tasks.push( f );
}
...
- 13
0
votes
3
answers
251
views
How do I create an std::array of immutable structs? I.e. structs with only const values [duplicate]
Say I have struct S:
struct S {
const int i;
const bool b;
}
And I want to create an (non-const) array of this struct like so:
std::array<S, 16> arr;
for(int i = 0; i<arr.size(); i++)
...
- 1,044
0
votes
1
answer
113
views
How to cast struct of std:array<char> to single std::array<char>
I'm packing messages to be send to another device. I'm using C++11. Usually the payload is a single ASCII encoded value, but this one command has a bit more parameters. I'm trying to pack the payload ...
- 1,044
0
votes
1
answer
468
views
Showing the unlock from std::condition_variable::wait
I've read from https://en.cppreference.com/w/cpp/thread/condition_variable/wait that wait() "Atomically unlocks lock". How do I see this via std::cout? I am trying to understand conditional ...
- 191
1
vote
1
answer
134
views
How can std::reference_wrapper<int> use operator+= if std::reference_wrapper doesn't have operator+=?
Thank you all, I didn't even know about user-defined conversion function and how it works.
Why is it possible to use std::reference_wrapper<int>::operator+=, if such an operator does not exist, ...
- 33
0
votes
0
answers
37
views
Correct way to pass std::vector from class to external function [duplicate]
I am attempting to pass a std::vector of a custom class datatype to an function external to the main class. However, I am not sure of the best way to do this. Currently it produces segmentation faults....
- 1
0
votes
0
answers
115
views
What does the syntax struct C{int x;}var; mean in C++ [duplicate]
I am looking at the documentation about std::for_each. (https://cplusplus.com/reference/algorithm/for_each/)
I cannot understand the way they are using the struct myclass because they are not ...
- 1
0
votes
2
answers
92
views
Initializing a 2-D array in C++14 [duplicate]
recently I've started to study C++'s standard library containers from the Deitel&Deitel book. Conceptually, it is quite clear to me, but I have a problem when trying to replicate a piece of code ...
- 1
1
vote
1
answer
939
views
What is the type of map's iterator?
I am new to C++. I found that to see a variable's type, I can use typeid().name() on library std::typeinfo.
But when I implement this function on map data structure, I got this output
Type of itr is :
...
- 115
0
votes
1
answer
288
views
C++ algorithm find_if in map
I want to find pair in map and just print it.
This is my code:
std::map<std::string, std::string> mLoginPasswordMap{ "test", "test2" };
std::string key1("test&...
- 11
1
vote
1
answer
64
views
Is it legal to derive from std::initializer_list?
Can I use std::initializer_list list as a base class?
template <typename T>
struct il : std::initializer_list<T>
{
using base = std::initializer_list<T>;
il(base list): base(...
- 9,095
1
vote
2
answers
550
views
Why is std::move generating instructions here?
I heard time and again that std::move(t) is more or less only a fancy way of saying static_cast<T&&>(t) and would not generate any instructions.
When I was playing around now with std::...
- 1,496
7
votes
1
answer
821
views
Indirect & Direct initialization of std::atomic in C++11/17. What are the differences?
On this CPP Con 2017 webinar, Fedor Pikus says: "it has to be direct initialization"
This is the link to the webinar.
What are the differences between these initialization methods? (and ...
- 1,130
0
votes
2
answers
795
views
Stopping multiple threads at once
What do I miss in the program below with threads waiting for a condition_variable_any to determine when to stop ? In the program listed below, the threads stop in an impredictable way; some before the ...
- 589
0
votes
0
answers
562
views
How to instantiate a future created by std::async in a member variable?
What do I do wrong when I try below to add the future returned by std::async into a member variable?
class PrinterDriver
{
std::vector<std::future<void>> m_PendingFutures;
public:
...
- 589
0
votes
1
answer
548
views
Why unique_ptr's Deleter need an argument of type unique_ptr<T, Deleter>::pointer?
I see the describtion about unique_ptr on cppreference, it says Deleter must be FunctionObject or lvalue reference to a FunctionObject or lvalue reference to function, callable with an argument of ...
- 52
0
votes
1
answer
210
views
"pair" class made from "std::tuple"
I know that "std::pair" struct does not inherit from "std::tuple" class.
However, I wanted to make "pair" class which inherited from "std::tuple" class.
Is this ...
- 11
0
votes
0
answers
70
views
find_if to seach abstract object list with std::string
I have a list which contains abstract objects and now I need to use a string search to the list to compare object's name and the string. Here is the abstract class.
class ShaderEffect {
std::string ...
- 433
4
votes
4
answers
853
views
Should a boolean predicate for std::condition_variable be volatile in C++?
I've been hearing so many conflicting answers, and now I don't know what to think.
The agreed-upon knowledge is that for sharing memory in a thread safe manner in C++, it's required to use volatile ...
- 301
0
votes
1
answer
636
views
How to pass reference type to std::hash
I am creating a Template Cache library in C++-11 where I want to hash the keys. I want to use default std::hash for primitive/pre-defined types like int, std::string, etc. and user-defined hash ...
1
vote
1
answer
115
views
Is std::lower_bound free from race conditions
Is it safe to run std::lower_bound on a shared vector object in an OpenMP loop? (using C++11)
Here is a basic example (edited from my code, not sure if it actually runs or not)
// fill range
std::...
- 297
0
votes
2
answers
162
views
Error while concatenating a vector to itself in c++
I am simply trying to concatenate a vector to itself but the following code is not working and I am not able to find the issue. If my input vector is {1,2,1}, the o/p I am getting is {1,2,1,1,16842944,...
- 369
3
votes
1
answer
175
views
Is there a way to delete from a set and move the value to a temporary?
Is there a way to delete an item in a set / map and get the former value as a temporary return value so that I can move() somewhere else ?
- 3,061
0
votes
2
answers
531
views
std:atomic initialized with reference creates copy of underlying variable?
Let's consider next snippet:
int val=5;
int& ref=val;
std::atomic<int> atomicref(ref);
++atomicref;
std::cout<< "atomic ref="<<atomicref.load()<<" original ...
- 689
5
votes
2
answers
499
views
Move a std::map vs move all the elements of a std::map
std::map<int, Obj> mp;
// insert elements into mp
// case 1
std::map<int, Obj> mp2;
mp2 = std::move(mp);
// case 2
std::map<int, Obj> mp3;
std::move(std::begin(mp), std::end(mp), ...
- 12.2k
2
votes
3
answers
4k
views
std::vector and move semantics
To enable move semantics on a std::vector do I need to pass by value; so the compiler will not copy the element but just move them?
Example:
class data
{
public:
void setMyData(vector<string>...
- 507
1
vote
1
answer
105
views
Why should we use std name space before functions of <algorithm> header and should not use it before functions of <cmath> header?
I was wondering why we should use the std namespace before function of <algorithm> header like max() and we are not obliged to use it before functions of <cmath> header like round()?
...
- 13
0
votes
1
answer
563
views
Using find_if with a vector of pointers: How to pass pointer by const reference to lambda?
In the following code I try to compare a vector of pointers via find_if and determine which contains a member a == 5 (in this case both of course, but it shows my case). However it doesn't compile.
#...
- 4,660
0
votes
2
answers
565
views
Is it possible to place a std::array at a fixed memory address?
For an embedded design I want to place a C++ std::array at a specific memory address, which points to a buffer shared by hardware and software. Is this possible?
- 2,359
2
votes
1
answer
148
views
How to delete the selected random data in std::vector?
How to delete the selected random data in vector?
#include <iostream>
#include <vector>
int main()
{
std::vector<int> shuf{ 1, 2, 3, 4, 5 };
int random = shuf[rand() % shuf....
- 144
1
vote
0
answers
60
views
Segmentation fault when applying std::sort to a vector of vectors of integers [duplicate]
I am writing a struct for sorting generic data types allowing for multiple values of the same in each object to be sorted (e.g. sorting a vector of vectors of integers). The code below is a simplified ...
- 288
1
vote
2
answers
211
views
How to make std::map::find function case sensitive?
I had interviewed with a MNC company. He gave me the following code and asked me to make find() function as case-sensitive. I tried, but failed to understand how to make inbuilt find function as case-...
- 4,893
12
votes
2
answers
781
views
Why is unlocking an unlocked std::mutex UB?
Unlocking a std::mutex that wasn't locked is UB. Why is this so? Why doesn't it just have no effect, as the mutex isn't locked yet, or was already unlocked, so what's the harm of calling unlock again?
- 249
0
votes
3
answers
450
views
C++ Conditional typedef with more than two type
I am trying to do something like this .
if (flow1)
{
typedef template_class<user_defined_type1> x;
}
else if (flow2)
{
typedef template_class<user_defined_type2> x;
}
...
- 223