>> Why?
Why not? It's concievable that there would be a default precision for the stream, one that would be valid everywhere so that it could be portable. It just so happens that that's the case. The default is 6:
#include <iostream>
int main()
{
double d = 123.4567890123;
std::cout << "Precision: " << std::cout.precision() << '\n';
std::cout << "Value: " << d << '\n';
std::cout.precision(4);
std::cout << "Value: " << d << '\n';
}
>> i want it to be say for example 8 or 10 decimal places
Assuming your system is capable of holding an accurate precision to 10 digits past the radix including an arbitrary number of digits before the radix, you would do it like this:
#include <iostream>
int main()
{
double x = 2.3767553235;
std::cout.precision(11);
std::cout << x << '\n';
}
However, because the precision is
all digits in the number, to get an exact precision of 10 for any whole value, you would need a way to determine the number of digits before the radix. An easy to understand solution is to make a string and take its length:
#include <iostream>
#include <sstream>
namespace jrd {
int digits(double val)
{
std::ostringstream oss;
oss << static_cast<int>(val);
return oss.str().length();
}
}
int main()
{
double x = 2.3767553235;
std::cout.precision(10 + jrd::digits(x));
std::cout << x << '\n';
}
Unsolved 
