On Sat, 21 Apr 2012, niXman wrote:
where is a code that compiles and works as expected:
#include <atomic>
#include <iostream>
int main() {
std::atomic<int> i(0);
i += 3;
std::cout << i << std::endl;
}
http://liveworkspace.org/code/eb92d1706687414dd3f49976584ff337
now change it to type double:
#include <atomic>
#include <iostream>
int main() {
std::atomic<double> i(0);
i += 3.14;
std::cout << i << std::endl;
}
http://liveworkspace.org/code/555191936a23b06352220ff11b1e33df
and get the following error:
source.cpp: In function 'int main()':
source.cpp:6:9: error: no match for 'operator+=' in 'i += 3.1400000000000001e+0'
therefore, I have two questions:
1. сan double be actually an atomic type?
Yes, because it is trivially copyable.
2. how is supposed to use atomic <double> by standard?
double isn't an integral type, so you can't do much more than store, load,
exchange and compare_exchange_{weak,strong}. For your case, you get the
double value from the atomic, increment it, then use compare_exchange to
store that new value in the atomic *if the atomic hasn't changed in the
meantime*, and if the atomic has changed, try again. Note that the
comparison is done with memcmp, not ==, which for double could be
different.
example from the standard:
expected = current.load();
do {
desired = function(expected);
} while (!current.compare_exchange_weak(expected, desired));
Note that there are already books available to learn these things, search
for instance for "C++ Concurrency in Action".
--
Marc Glisse
