I know it is a repeat question but non of the answers that I read is addressing the issue. Hence I am Posting the question again.
I wrote this very simple program
#include<iostream>
#include<thread>
using namespace std;
void func1(char ch){
for(int i=0; i<100; i++){
cout<<ch;
}
}
int main(){
std::thread worker1(func1, 'a');
std::thread worker2(func1, 'b');
getchar();
return 0;
}
The above code when compiler with (MinGW.org GCC-6.3.0-1) 6.3.0(C++ 14) gives me error as thread' is not a member of 'std'. However, I am able to go to the definition of thread at location C:\MinGW\lib\gcc\mingw32\6.3.0\include\c++\thread. The same code when compiled on Ubuntu with compiler g++ (Ubuntu 11.3.0-1ubuntu1~22.04) 11.3.0 works perfectly fine. I don't seem to find a reason why it should not work on Windows.
Related
I've installed MinGW(gcc 6.3.0) and Eclipse 4.7 on Windows 8.1. I could compile and run other programs without any issue. But when I was using thread it is showing that thread was not declared in this scope. I've tried this and this but both no avail. Could somebody help me fix this?
Here is the code I've treid.
#include <iostream>
#include <thread>
void foo(){
std::cout<<"thread function \n";}
int main(void)
{
std::thread t1(foo);
t1.join();
return 0;
}
I'm not sure about the C++11 features. I've tried Lambda function example from this page(last example) and it is working fine.
my thread program is :
#include<iostream>
#include<thread>
using namespace std;
void t()
{
cout<<"from thread\n";
}
int main()
{
thread i(&t);
cout <<"from main\n";
i.join();
}
but it shows following error in codeblocks:
1)'thread ' was not declared in this scope
2)expected ';' before 'i'
3)'i' was not declared in this scope
How can I solve it?I am using windows and codeblocks 12.11
I have the same problem. Unfortunately the version of GCC Code::Blocks uses does not support the features of C++11 that you want. Turning on the option -std=c++0x will only ugrade the compiler to a less recent version of the new Standard. That means you will only enable basic support of C+11.
My thread program is:
#include<iostream>
#include<thread>
using namespace std;
void t()
{
cout<<"from thread\n";
}
int main()
{
thread i(&t);
cout <<"from main\n";
i.join();
}
but it shows following error in codeblocks:
1)'thread ' was not declared in this scope
2)expected ';' before 'i'
3)'i' was not declared in this scope
How can I solve it?I am using windows and codeblocks 12.11
First, are you on windows or linux?
If you are on linux, you must compile with C++11 support. Just pass -std=c++11 to g++.
I can't help you with windows.
Your IDE may not support C++11 yet. since thread is included in the standard since C++11. See this thread for CodeBlocks? http://forums.codeblocks.org/index.php?topic=15536.0
I try to use C++11 theading library using g++ 4.7.
First I have a question: is it expected for a next release to not be required to link by hand the pthread library ?
So my program is :
#include <iostream>
#include <vector>
#include <thread>
void f(int i) {
std::cout<<"Hello world from : "<<i<<std::endl;
}
int main() {
const int n = 4;
std::vector<std::thread> t;
for (int i = 0; i < n; ++i) {
t.push_back(std::thread(f, i));
}
for (int i = 0; i < n; ++i) {
t[i].join();
}
return 0;
}
I compile with:
g++-4.7 -Wall -Wextra -Winline -std=c++0x -pthread -O3 helloworld.cpp -o helloworld
And it returns:
Hello world from : Hello world from : Hello world from : 32
2
pure virtual method called
terminate called without an active exception
Erreur de segmentation (core dumped)
What is the problem and how to solve it ?
UPDATE:
Now using mutex:
#include <iostream>
#include <vector>
#include <thread>
#include <mutex>
static std::mutex m;
void f(int i) {
m.lock();
std::cout<<"Hello world from : "<<i<<std::endl;
m.unlock();
}
int main() {
const int n = 4;
std::vector<std::thread> t;
for (int i = 0; i < n; ++i) {
t.push_back(std::thread(f, i));
}
for (int i = 0; i < n; ++i) {
t[i].join();
}
return 0;
}
It returns :
pure virtual method called
Hello world from : 2
terminate called without an active exception
Abandon (core dumped)
UPDATE 2:
Hum... It works with my default GCC (g++4.6) but it fails with the version of gcc I compiled by hand (g++4.7.1). Was there an option I forgot to compile g++ 4.7.1 ?
General edit:
In order to prevent use of cout by multiple threads simultaneously, that will result in character interleaving, proceed as follows:
1) declare before the declaration of f():
static std::mutex m;
2) then, guard the "cout" line between:
m.lock();
std::cout<<"Hello world from : "<<i<<std::endl;
m.unlock();
Apparently, linking against the -lpthread library is a must, for some unclear reasons. At least on my machine, not linking against -lpthread results in a core dump. Adding -lpthread results in proper functionality of the program.
The possibility of character interleaving if locking is not used when accessing cout from different threads is expressed here:
https://stackoverflow.com/a/6374525/1284631
more exactly: "[ Note: Users must still synchronize concurrent use of these objects and streams by multiple threads if they wish to avoid interleaved characters. — end note ]"
OTOH, the race condition is guaranteed to be avoided, at least in the C++11 standard (beware, the gcc/g++ implementation of this standard is still at experimental level).
Note that the Microsoft implementation (see: http://msdn.microsoft.com/en-us/library/c9ceah3b.aspx credit to #SChepurin) is stricter than the standard (apparently, it guarantees character interleaving is avoided), but this might not be the case for the gcc/g++ implementation.
This is the command line that I use to compile (both updated and original code versions, everything works well on my PC):
g++ threadtest.cpp -std=gnu++11 -lpthread -O3
OTOH, without the -lpthread, it compiles but I have a core dump (gcc 4.7.2 on Linux 64).
I understand that you are using two different versions of the gcc/g++ compiler on the same machine. Just be sure that you use them properly (not mixing different library versions).
When was the unordered_map concept built into g++?
Because the following code throws an error.
#include<iostream>
#include<unordered_map>
#include<stdio.h>
using namespace std;
std::unordered_map<std::int,int> mirror;
mirror['A'] = 'A';
mirror['B'] = '#';
mirror['E'] = 3;
int main(void)
{
std::cout<<mirror['A'];
std::cout<<mirror['B'];
std::cout<<mirror['C'];
return 0;
}
I am compiling the code as follows:
g++ -c hashexample.cpp
g++ -o result hashExample.o
./result
The error I got is this:
inavalid types int[char[ for aaray subscript
What is the fix for this?
The problem is your assignment. You cannot assign values to your map in this place. C++ is not a script language.
This program works fine on my machine with gcc4.6:
#include<iostream>
#include<unordered_map>
std::unordered_map<int,int> mirror;
int main() {
mirror['A'] = 'A';
mirror['B'] = '#';
mirror['E'] = 3;
std::cout<<mirror['A'];
std::cout<<mirror['B'];
std::cout<<mirror['C'];
}
First, as mkaes points out, you cannot put assignments outside functions, so you have to put it in any, for example main.
As for unordered_map, for recent versions of gcc, if you don't want to go into C++11, you can use the TR1 version of unordered_map:
#include <tr1/unordered_map>
and the type std::tr1::unordered_map. You know, C++11 supersedes all this, but you will (at least in GCC) get this working.