C++ error: initial value of reference to non-const must be an lvalue

//实例1
#include
using namespace std;
int add(int& a,int& b)
{
int sum = a + b;
return sum;
}
int main()
{
int i =sum(6,7);
cout<
using namespace std;
int add(const int& a,const int& b)
{
int sum = a + b;
return sum;
}
int main()
{
int i =sum(6,7);
cout<

#分享C++学习中遇到的困难。