Integers are inherently finite. The closest you can get is by setting a to int‘s maximum value:
#include <limits>
// ...
int a = std::numeric_limits<int>::max();
Which would be 2^31 - 1 (or 2 147 483 647) if int is 32 bits wide on your implementation.
If you really need infinity, use a floating point number type, like float or double. You can then get infinity with:
double a = std::numeric_limits<double>::infinity();