第一种方法：cout<<fixed<<setprecision(20)<<mydouble<<endl;

#include 
    
     #include 
     
      using namespace std;int main(){	double aDouble = 5.141592694827862736487362746374637434343434;	cout<
      
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第二种方法：cout.precision(dbl::digits);

#include 
    
     using namespace std;int main(){	typedef numeric_limits< double > dbl;	double pi = 3.141592694827862736487362746374637434343434;	cout.precision(dbl::digits10);	cout << "Pi: " << fixed << pi << endl;	return 0;}