\alpha=arcsin\sqrt{n^2-\frac{\lambda^2}{16d^2}}

Here's how you can implement the given equation in C++:

```cpp
#include <cmath>
#include <iostream>

// Function to calculate alpha
double calculateAlpha(double n, double lambdaValue, double d) {
    // Check if the input values are valid
    if (n < 0 || lambdaValue < 0 || d < 0) {
        throw std::invalid_argument("Input values must be non-negative.");
    }

    // Calculate alpha
    double alpha = asin(sqrt(n * n - lambdaValue * lambdaValue / (16 * d * d)));

    return alpha;
}

int main() {
    double n = 10.0;  // Example value for n
    double lambdaValue = 5.0;  // Example value for lambda
    double d = 2.0;  // Example value for d

    try {
        double alpha = calculateAlpha(n, lambdaValue, d);
        std::cout << "Alpha: " << alpha << std::endl;
    } catch (const std::exception& e) {
        std::cerr << "Error: " << e.what() << std::endl;
        return 1;
    }

    return 0;
}
```

This code defines a function `calculateAlpha` that takes three arguments: `n`, `lambdaValue`, and `d`. It calculates `alpha` using the given equation and returns the result. The `main` function demonstrates how to use this function with example values for `n`, `lambdaValue`, and `d`. It also includes error handling to catch any invalid input values.