Consider the declaration of a class that contains a lambda function or a reference to it:
#include <utility>
template <class Func>
struct Holder
{
Holder(Func && func) : m_func(std::forward<Func>(func))
{
}
Func m_func;
};
template <class Func>
auto MakeHolder(Func && func)
{
return Holder<Func>(std::forward<Func>(func));
}
I did some research on why my QT app crashes at the destructor of std::thread on Android 10 devices at the user side with the following call stack:
#00 /apex/com.android.runtime/lib64/bionic/libc.so (abort+160)
#01 /system/lib64/libc++.so (abort_message+232)
#02 /system/lib64/libc++.so (demangling_terminate_handler()+44)
#03 /system/lib64/libc++.so (std::__terminate(void (*)())+12)
#04 /system/lib64/libc++.so (std::terminate()+52)
#05 /system/lib64/libc++.so (std::__1::thread::~thread()+20)
#06 /apex/com.android.runtime/lib64/bionic/libc.so (__cxa_finalize+212)
#07 /apex/com.android.runtime/lib64/bionic/libc.so (exit+24)
#08 /data/app/com.domain.myapp-Rs_sm5VrLR1Jj8QW6oYByA==/lib/arm64/libplugins_platforms_qtforandroid_arm64-v8a.so
and figured out that its likely because std::thread destructor is being invoked while the thread is still joinable at some point of the application execution (thanks to G. M. on stackoverflow.com).
(more…)I wrote a simple test that outputs std::memmove speed to the console:
AWT_ATTRIBUTE(size_t, element_count, 1000000);
std::unique_ptr<uint8_t> p_src(new uint8_t[element_count]);
std::memset(p_src.get(), 25u, element_count);
std::unique_ptr<uint8_t> p_dst(new uint8_t[element_count]);
context.out << _T("std::memmove: ");
awl::StopWatch w;
std::memmove(p_dst.get(), p_src.get(), element_count);
ReportSpeed(context, w, element_count);
context.out << std::endl;
And the similar tests for std::memset and std::vector::insert.
(more…)In the code below, static_assert operates on the addresses of local variables, but however MS compiles it and the assertion does not fail:
#include <tuple>
#include <string>
struct A
{
int a;
double b;
std::string c;
};
void f()
{
A a{ 1, 3.0, "abc" };
constexpr auto t1 = std::tie(a.a, a.b, a.c);
static_assert(&std::get<0>(t1) == &a.a);
}
Use the following command to compile this example:
cl /std:c++17 /EHsc a.cpp
Consider the following scenario: there was structure A in an old version of a C++ application:
struct A
{
double a;
int b;
std::string c;
};
An instance of A was serialized into a file in a binary format and after that the application was updated to a new version.
But in the new version of the application structure A was modified by adding fields d and e and deleting field a:
struct A
{
int b;
std::vector<int> d;
bool e;
std::string c;
};
and the new version of the application needs to deserialize an instance of its new structure A from the file containing old version of A.
(more…)I implemented a simple C++ binary serialization framework that makes a structure or class serializable by adding a macro that usually takes one line of code as shown in the example below:
struct A
{
bool x;
int y;
AWL_SERIALIZABLE(x, y)
};
There is also a macro that makes a structure or a class equatable:
AWL_MEMBERWISE_EQUATABLE(A)
The code below demonstrates how to parse GUID in C++ using a regular expression:
#include <iostream>
#include <string>
#include <regex>
int main()
{
static const std::wregex regex(L"[0-9a-fA-F]{8}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{12}");
std::wstring sample = L"850fe1da-0ea6-c1a8-9810-0c1cece30698";
std::match_results<std::wstring::const_iterator> match;
if (std::regex_match(sample, match, regex))
{
std::wcout << L"matches" << std::endl;
}
else
{
std::wcout << L"does not match" << std::endl;
}
return 0;
}
For example, std::forward comes to play if I implement a template container with two ‘insert‘ function overloads that take the parameters of type const T & and T && respectively. I implement all the private insertion logic with T&&, but when I need to know the original value type passed to ‘insert‘ method I use std::forward as shown in the example below:
#include <iostream>
#include <string>
class Value
{
public:
Value(const char * sz) : m_s(sz)
{
}
Value(const Value & other) : m_s(other.m_s)
{
std::cout << "copy constructor: " << m_s << std::endl;
}
Value(Value && other) : m_s(std::move(other.m_s))
{
std::cout << "move constructor: " << m_s << std::endl;
}
const std::string & ToString() const
{
return m_s;
}
private:
std::string m_s;
};
The string representation of a type is implementation defined in C++, for example the following code produce the different output with MSVC, GCC and CLang:
#include <string>
#include <iostream>
struct A {};
class B {};
namespace ns
{
struct X {};
}
int main()
{
std::cout << typeid(A).name() << ", " << typeid(B).name() << ", " << typeid(ns::X).name() << ", " << typeid(std::string).name() << std::endl;
return 0;
}
We had a discussion with colleagues on why in the following code we cannot simply use
static_assert(false)
but need to do a trick with ‘always_false’:
#include <type_traits>
template<typename>
struct always_false : std::false_type {};
template<typename Type>
constexpr int Get()
{
if constexpr (std::is_same_v<Type, int>)
{
return 1;
}
else if constexpr (std::is_same_v<Type, bool>)
{
return 2;
}
else {
static_assert(always_false<Type>::value);
}
}
