I have a problem compiling code using std::variant.
I try to compile this code with g++ 5.4/6.2 on ubuntu and fedora with -std=c++17:
#include <variant>
#include <string>
int main()
{
std::variant<int, float> v, w;
v = 12; // v contains int
int i = std::get<int>(v);
w = std::get<int>(v);
w = std::get<0>(v); // same effect as the previous line
w = v; // same effect as the previous line
try {
std::get<float>(w); // w contains int, not float: will throw
}
catch (std::bad_variant_access&) {}
std::variant<std::string> v("abc"); // converting constructors work when unambiguous
v = "def"; // converting assignment also works when unambiguous
}
found on cppreference.com but this error append: "fatal error: variant: No such file or directory".
Install g++-7; e.g. if ubuntu
sudo add-apt-repository ppa:ubuntu-toolchain-r/test -y
sudo apt-get update
sudo apt-get install g++-7
run /usr/bin/g++-7 -std=c++1z your_program.cc
Optional, if you use cmake system, add these lines :
set(CMAKE_CXX_COMPILER "/usr/bin/g++-7")
set(CMAKE_CXX_FLAGS "-std=c++1z")
std::variant was added in C++17.
gcc does not yet fully support the relevant bits of the C++17 standard.
I don't even see std::variant listed in the gcc's tracking page.
Related
My system: macOS Catalina(10.15.4)
Brew config
Clang: 11.0 build 1103
Git: 2.21.0 => /usr/local/bin/git
Curl: 7.64.1 => /usr/bin/curl
Java: 1.8.0_101
macOS: 10.15.4-x86_64
CLT: 11.4.1.0.1.1586360307
Xcode: N/A
Issue: can't include some C++17 headers: execution, filesystem
while can include the other like: any, variant.
Most of C++17 features like structured binding, template parameters deduction etc works.
Example. Trying to build a file with the following code snippet:
#include <numeric>
#include <vector>
#include <iostream>
#include <iterator>
#include <string>
#include <sstream>
#include <utility> //std::pair
#include <tuple> // std::tie
#include <algorithm> // std::clamp
#include <optional> // std::optional
#include <map>
// ... other stuff
#include <execution>
namespace parallel_algoritmhs {
void show() {
std::vector<int> v = {1,2,3,4,5,6};
std::for_each(std::execution::par, v.begin(), v.end(), [](auto& e) {e+=100;});
int main() {
//... use
}
I am building with:
g++ -std=c++17 -fsanitize=undefined -fno-sanitize-recover=all -o main c++17.cpp
Update:
I have tried suggested:
clang -std=c++17 -fsanitize=undefined -fno-sanitize-recover=all -o main c++17.cpp
clang++ -std=c++17 -fsanitize=undefined -fno-sanitize-recover=all -o main c++17.cpp
And getting the same result.
If I comment out code snippet related to different execution strategies, while leaving other code snippets like:
#include <any>
namespace any {
void show() {
std::any v = 42;
v = 4.2;
v = std::string{"hello"};
std::cout << std::any_cast<std::string>(v) << '\n';
}
}
namespace map_cpp17 {
void show() {
std::map<int, std::string> myMap{ { 1, "Gennady" }, { 2, "Petr" }, { 3, "Makoto" } };
auto node = myMap.extract(2);
node.key() = 42;
myMap.insert(std::move(node));
std::map<int, std::string> m1{ { 1, "aa" }, { 2, "bb" }, { 3, "cc" } };
std::map<int, std::string> m2{ { 4, "dd" }, { 5, "ee" }, { 6, "ff" } };
m1.merge(m2);
std::map<int, std::string> m; m.emplace(1, "aaa"); m.emplace(2, "bbb"); m.emplace(3, "ccc");
auto [it1, inserted1] = m.insert_or_assign(3, "ddd"); std::cout << inserted1; // 0
auto [it2, inserted2] = m.insert_or_assign(4, "eee"); std::cout << inserted2; // 1
}
}
Everything compiles
With
#include <execution>
getting:
fatal error: 'execution' file not found
#include <execution>
I wonder what might be the problem behind that and how to fix this.
Thank you!
Update 2:
brew info llvm:
llvm: stable 10.0.0 (bottled), HEAD [keg-only]
Next-gen compiler infrastructure
https://llvm.org/
Not installed
From: https://github.com/Homebrew/homebrew-core/blob/master/Formula/llvm.rb
==> Dependencies
Build: cmake ✔, python#3.8 ✘
Required: libffi ✘
==> Requirements
Build: xcode ✘
==> Options
--HEAD
Install HEAD version
==> Caveats
To use the bundled libc++ please add the following LDFLAGS:
LDFLAGS="-L/usr/local/opt/llvm/lib -Wl,-rpath,/usr/local/opt/llvm/lib"
llvm is keg-only, which means it was not symlinked into /usr/local,
because macOS already provides this software and installing another version in
parallel can cause all kinds of trouble.
Update 3:
clang --version
Apple clang version 11.0.3 (clang-1103.0.32.59)
Target: x86_64-apple-darwin19.4.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin
You're not actually using the Clang you installed. The official LLVM Clang doesn't have a version 11.0! LLVM is only on version 10.0 (check brew info llvm). You're using Apple Clang, which comes with its own installation of libc++. Presumably to catch this exact type of problem, Apple Clang responds to clang --version/clang++ --version with the "future" version 11.0. However, Apple's distribution is actually a bit behind the state of the art, and it simply isn't updated to these features.
When you install LLVM with Homebrew, it doesn't automatically set it up the new clang/clang++ to be easily callable, because things might expect clang to specifically mean "Apple Clang" and break if that changes. You should add /usr/local/opt/llvm/bin (which Homebrew symlinks into the LLVM installation) to your PATH (which is where your shell and other programs search for programs). In an already running shell session
export PATH="/usr/local/opt/llvm/bin:$PATH"
will switch to using the newly installed LLVM Clang until you exit the session. Putting this line in a shell startup file, like ~/.bash_profile, will set up this adjustment to be applied every time you open a new shell.
Alternatively, like #Eljay does, you could just manually type out the full path to the new clang/clang++ (/usr/local/opt/llvm/bin/clang/-++), thereby bypassing the search into PATH, but that's a pain. In any case, you'll know you got it right if clang --version/clang++ --version gives you 10.0.0.
I met a strange behavior when using add_property of Boost.Python.
Here is the example codes, it simply add two properties (read-only & read-write) to the wrapped class.
#include <boost/python.hpp>
using namespace boost::python;
struct X {
X( int value ) : val( value ) {}
int get_value() const
{ return val; }
void set_value(int value)
{ val = value; }
int val;
};
BOOST_PYTHON_MODULE(hello) {
class_<X>("X", init<int>() )
// NOTE the order
.add_property( "val_r", &X::get_value ) // read-only first
.add_property( "val_rw", &X::get_value, &X::set_value ) // read-write second
;
}
and the build command:
$ g++ -I/usr/include/python2.7 -shared -L/usr/local/lib -o bp.dll bp.cpp -lboost_python -lpython2.7
I'm working in Cygwin x86_64, so the file extension is .dll. As for Ubuntu just change the output extension to .so.
When importing the bp module, it crashes with a Segmentation fault. The core dump file doesn't contain any call stack information.
However, if I change the order of add_property (as below) to add the read-write property first, it works ok.
.add_property( "val_rw", &X::get_value, &X::set_value ) // read-write first
.add_property( "val_r", &X::get_value ) // read-only second
My environment:
Cygwin x86_64
python 2.7.10
gcc 5.4.0
boost: 1.60.0
BTW, there's no problem in Ubuntu 14.04 (64-bit) whatever the add_property order is.
Why would this happen?
I simply want to match "{". But don't know why giving this error:
terminate called after throwing an instance of 'std::regex_error'
what(): regex_error
Aborted (core dumped)
Compilation on Ubuntu with g++ version 4.6.3
g++ -std=c++0x a.c
Program
#include<iostream>
#include<regex>
using namespace std;
main(int argc,char**argv){
if (regex_match("{1}" , std::regex ("[{]"))) {
cout<<"Hello World"<<endl;
}
}
I've also checked the ECMAScript details and this regular expression should match. It also does not match when I use like : std::regex ("\\{"))
What am I wrong?
You need at least gcc 4.9 to make regexps work with gcc, once you will have 4.9 version add .* to make it match the rest of the string:
if (regex_match("{1}" , std::regex ("[{].*"))) {
^^
http://coliru.stacked-crooked.com/a/99e405e66906804d
I have the same error with you! And my IDE is Clion, I choose the C++ version with Clion is C++17 and my test code is:
std::string pattern{ "http|hppts://\\w.*$" }; // url
std::regex re(pattern);
std::vector<std::string> str{ "http://blog.net/xxx",
"https://github.com/jasonhubs", "abcd://124.456",
"abcdhttps://github.com/jasonhubs"
};
for (auto tmp : str)
{
bool ret = std::regex_search(tmp, re);
if (ret) fprintf(stderr, "%s, can search\n", tmp.c_str());
else fprintf(stderr, "%s, can not search\n", tmp.c_str());
}
and I solve it by updating the gcc and g++.
sudo yum install centos-release-scl yum-utils
sudo yum-config-manager --enable rhel-server-rhscl-7-rpms
sudo yum install devtoolset-7
scl enable devtoolset-7 bash
I read it here that CUDA 6.5 has started support for C++11 :
https://groups.google.com/forum/#!topic/thrust-users/R37GIkMG4tk
But when I compile an example code below, I got
$ nvcc -std=c++11 cu-gcc11.cu -o test
nvcc warning : The -c++11 flag is not supported with the configured host compiler. Flag will be ignored.
cu-gcc11.cu(7): error: explicit type is missing ("int" assumed)
My setting : CUDA 6.5, g++ 4.5, ubuntu 12.04
Codes :
#include <cuda.h>
#include <iostream>
__host__ void test() {
float a = 12.;
double b = 3.;
auto c = a * b;
std::cout << c << std::endl;
}
int main()
{
test();
return 0;
}
C++11 support in nvcc is experimental at this time. In order to properly use it you will need an appropriate configuration. This is not documented anywhere AFAIK, but you should have good results with either Fedora 20 or Ubuntu 14.04, both of which are supported configs for cuda 6.5 and include GCC 4.8.x.
In your case your GCC version is just too old.
I don't think -std=c++11 was available in GCC 4.5. Try -std=c++0x.
I'm having trouble getting the library working on macosx. First off, I tried to compile the following code, saved as rand.cpp, taken from the c++ website
#include <iostream>
#include <random>
int main()
{
const int nrolls=10000; // number of experiments
const int nstars=100; // maximum number of stars to distribute
std::default_random_engine generator;
std::normal_distribution<double> distribution(5.0,2.0);
int p[10]={};
for (int i=0; i<nrolls; ++i) {
double number = distribution(generator);
if ((number>=0.0)&&(number<10.0)) ++p[int(number)];
}
std::cout << "normal_distribution (5.0,2.0):" << std::endl;
for (int i=0; i<10; ++i) {
std::cout << i << "-" << (i+1) << ": ";
std::cout << std::string(p[i]*nstars/nrolls,'*') << std::endl;
}
return 0;
}
Upon running this with g++ rand.cpp -o rand i get the following errors
rand.cpp:9: error: ‘default_random_engine’ is not a member of ‘std’
rand.cpp:10: error: ‘normal_distribution’ is not a member of ‘std’
Searching around it seems to be suggested that the issue is the compiler, apparently thus library is only available to gcc11. I found a way to update gcc using the macport package as shown here Update GCC on OSX but I still don't know how to use this new compiler. Running g++ rand.cpp -o rand returns the same errors even when I change the compiler with sudo port select --set gcc gcc40 or sudo port select --set gcc mp-gcc46. I also tried using g++ -std=c++11 rand.cpp -o rand which just returns
cc1plus: error: unrecognized command line option "-std=c++11"
Does anyone know what I am doing wrong?
Try it with Clang++, which should be installed in your mac, or a new version of GCC.
gcc42: I had this version installed, it didn't work, and didn't recognize -std=c++0x and -std=c++11.
gcc49: Installed this with brew, it gave the same error but -std=c++11 made it work.
Clang++: Worked like a charm without even specifying the standard (it probably defaults to c++11).
Also, check if you have the latest version of the command line tools, if not, download them from the Downloads for Apple Developers website.
What you're doing wrong
The version you installed doesn't have the -std=c++11 option, but it should work with -std=c++0x or -std=gnu++0x, that's what it says in the manual for the 4.6 version.