[c++] What is the meaning of "operator bool() const"

operator bool() const 
{
    return col != 0;
}

Defines how the class is convertable to a boolean value, the const after the () is used to indicate this method does not mutate (change the members of this class).

You would usually use such operators as follows:

airplaysdk sdkInstance;
if (sdkInstance) {
    std::cout << "Instance is active" << std::endl;
} else {
    std::cout << "Instance is in-active error!" << std::endl;
}

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What is the meaning of "operator bool() const"