如何使用 boost::blank

Question

I want to add empty values to int and double with the help of boost::blank . Later, I want to check if these values are empty. I have had a hard time finding information on the functionality of boost::blank . Can someone help with some examples of how to use boost::blank ?

Answer 1

boost::blank is not quite what you're looking for.

Here's what boost::blank is:

struct blank
{
};

// overloaded operators
// ...

It could be useful if you were creating a std::variant (or boost::variant ) and wanted an "empty variant" like so:

std::variant<boost::blank, int, double, char> my_variant;

If you want to add an "empty" value for simply an int or a double in isolation, you should use std::optional<int> ( std::optional<double> )

You can check for empty values with a std::optional like so:

std::optional<int> value;
std::cout << std::boolalpha << value.has_value() << std::endl; // "false"

It's also implicitly convertible to bool , so you can check for a value in an if like so:

if (value)
    std::cout << "Has a value: " << *value << std::endl; // branch not taken

And finally, you can ask to use a default if there's no value, like so:

std::cout << value.value_or(0) << std::endl; // "0"

Live Demo

---

If boost::blank is really what you're looking for, let's revisit the std::variant<blank, int, double> scenario.

Construct an "empty" variant:

std::variant<boost::blank, int, double> my_variant;

or equivalently,

std::variant<blank, int, double> my_variant = boost::blank{};

Next, define visitors for each type:

struct my_visitor
{
    void operator()(boost::blank) const
    {
        std::cout << "Variant is empty\n";
    }
    void operator()(int i) const
    {
        std::cout << "Variant has int with value " << i << std::endl;
    }
    void operator()(double d) const
    {
        std::cout << "Variant has double with value " << d << std::endl;
    }
};

And we can execute the appropriate function using std::visit :

// int
my_variant = 42;
std::visit(visitor, my_variant);

// double
my_variant = 1337.0;
std::visit(visitor, my_variant);

// reset to empty
my_variant = boost::blank{};
std::visit(visitor, my_variant);

You can also check for an "empty" variant by calling holds_alternative :

if (std::holds_alternative<blank>(my_variant))
    std::cout << "Variant is empty\n";

Demo with variant