In C++, you can check if a string is a valid number by using the stoi() and stod() functions. The stoi() function converts a string to an integer, while the stod() function converts a string to a double. If the string is not a valid number, these functions will throw an exception, which can be caught using a try-catch block. Here's an example:
``c++
#include
#include
using namespace std;
int main() {
string input;
cout << "Enter a number: ";
cin >> input;
try {
int number = stoi(input);
cout << "It's an integer!" << endl;
} catch (...) {
try {
double number = stod(input);
cout << "It's a decimal!" << endl;
} catch (...) {
cout << "It's not a number!" << endl;
}
}
return 0;
}
`
In this example, the program prompts the user to enter a number, reads it in as a string, and then tries to convert it to an integer using stoi(). If that fails (i.e. the string is not a valid integer), the program tries to convert the string to a double using stod()`. If that also fails, the program concludes that the string is not a number.