c++ - 限制 C++ 中值类型的范围

Question

假设我有一个 LimitedValue 类，它保存一个值，并在 int 类型“min”和“max”上进行参数化。您可以将它用作保存只能在一定范围内的值的容器。你可以这样使用它:

LimitedValue< float, 0, 360 > someAngle( 45.0 );
someTrigFunction( someAngle );

这样 'someTrigFunction' 就知道保证提供了一个有效的输入(如果参数无效，构造函数会抛出异常)。

但是，复制构造和赋值仅限于完全相同的类型。我希望能够做到:

LimitedValue< float, 0, 90 > smallAngle( 45.0 );
LimitedValue< float, 0, 360 > anyAngle( smallAngle );

并在编译时检查该操作，因此下一个示例给出错误:

LimitedValue< float, -90, 0 > negativeAngle( -45.0 );
LimitedValue< float, 0, 360 > postiveAngle( negativeAngle ); // ERROR!

这可能吗？有没有一些实用的方法可以做到这一点，或者有什么例子可以做到这一点？

Answer 1

好的，这是没有 Boost 依赖的 C++11。

在编译时检查类型系统保证的所有内容，其他任何内容都会引发异常。

我为可能抛出的转换添加了 unsafe_bounded_cast，为静态正确的显式转换添加了 safe_bounded_cast(这是多余的，因为复制构造函数处理它，但提供对称性和表现力)。

使用示例

#include "bounded.hpp"

int main()
{
    BoundedValue<int, 0, 5> inner(1);
    BoundedValue<double, 0, 4> outer(2.3);
    BoundedValue<double, -1, +1> overlap(0.0);

    inner = outer; // ok: [0,4] contained in [0,5]

    // overlap = inner;
    // ^ error: static assertion failed: "conversion disallowed from BoundedValue with higher max"

    // overlap = safe_bounded_cast<double, -1, +1>(inner);
    // ^ error: static assertion failed: "conversion disallowed from BoundedValue with higher max"

    overlap = unsafe_bounded_cast<double, -1, +1>(inner);
    // ^ compiles but throws:
    // terminate called after throwing an instance of 'BoundedValueException<int>'
    //   what():  BoundedValueException: !(-1<=2<=1) - BOUNDED_VALUE_ASSERT at bounded.hpp:56
    // Aborted

    inner = 0;
    overlap = unsafe_bounded_cast<double, -1, +1>(inner);
    // ^ ok

    inner = 7;
    // terminate called after throwing an instance of 'BoundedValueException<int>'
    //   what():  BoundedValueException: !(0<=7<=5) - BOUNDED_VALUE_ASSERT at bounded.hpp:75
    // Aborted
}

异常支持

这有点样板，但提供了如上所述的相当可读的异常消息(如果您选择捕获派生的异常类型并可以用它做一些有用的事情，那么实际的最小/最大值/值也会暴露出来)。

#include <stdexcept>
#include <sstream>

#define STRINGIZE(x) #x
#define STRINGIFY(x) STRINGIZE( x )

// handling for runtime value errors
#define BOUNDED_VALUE_ASSERT(MIN, MAX, VAL) \
    if ((VAL) < (MIN) || (VAL) > (MAX)) { \
        bounded_value_assert_helper(MIN, MAX, VAL, \
                                    "BOUNDED_VALUE_ASSERT at " \
                                    __FILE__ ":" STRINGIFY(__LINE__)); \
    }

template <typename T>
struct BoundedValueException: public std::range_error
{
    virtual ~BoundedValueException() throw() {}
    BoundedValueException() = delete;
    BoundedValueException(BoundedValueException const &other) = default;
    BoundedValueException(BoundedValueException &&source) = default;

    BoundedValueException(int min, int max, T val, std::string const& message)
        : std::range_error(message), minval_(min), maxval_(max), val_(val)
    {
    }

    int const minval_;
    int const maxval_;
    T const val_;
};

template <typename T> void bounded_value_assert_helper(int min, int max, T val,
                                                       char const *message = NULL)
{
    std::ostringstream oss;
    oss << "BoundedValueException: !("
        << min << "<="
        << val << "<="
        << max << ")";
    if (message) {
        oss << " - " << message;
    }
    throw BoundedValueException<T>(min, max, val, oss.str());
}

值类

template <typename T, int Tmin, int Tmax> class BoundedValue
{
public:
    typedef T value_type;
    enum { min_value=Tmin, max_value=Tmax };
    typedef BoundedValue<value_type, min_value, max_value> SelfType;

    // runtime checking constructor:
    explicit BoundedValue(T runtime_value) : val_(runtime_value) {
        BOUNDED_VALUE_ASSERT(min_value, max_value, runtime_value);
    }
    // compile-time checked constructors:
    BoundedValue(SelfType const& other) : val_(other) {}
    BoundedValue(SelfType &&other) : val_(other) {}

    template <typename otherT, int otherTmin, int otherTmax>
    BoundedValue(BoundedValue<otherT, otherTmin, otherTmax> const &other)
        : val_(other) // will just fail if T, otherT not convertible
    {
        static_assert(otherTmin >= Tmin,
                      "conversion disallowed from BoundedValue with lower min");
        static_assert(otherTmax <= Tmax,
                      "conversion disallowed from BoundedValue with higher max");
    }

    // compile-time checked assignments:
    BoundedValue& operator= (SelfType const& other) { val_ = other.val_; return *this; }

    template <typename otherT, int otherTmin, int otherTmax>
    BoundedValue& operator= (BoundedValue<otherT, otherTmin, otherTmax> const &other) {
        static_assert(otherTmin >= Tmin,
                      "conversion disallowed from BoundedValue with lower min");
        static_assert(otherTmax <= Tmax,
                      "conversion disallowed from BoundedValue with higher max");
        val_ = other; // will just fail if T, otherT not convertible
        return *this;
    }
    // run-time checked assignment:
    BoundedValue& operator= (T const& val) {
        BOUNDED_VALUE_ASSERT(min_value, max_value, val);
        val_ = val;
        return *this;
    }

    operator T const& () const { return val_; }
private:
    value_type val_;
};

Actor 支持

template <typename dstT, int dstMin, int dstMax>
struct BoundedCastHelper
{
    typedef BoundedValue<dstT, dstMin, dstMax> return_type;

    // conversion is checked statically, and always succeeds
    template <typename srcT, int srcMin, int srcMax>
    static return_type convert(BoundedValue<srcT, srcMin, srcMax> const& source)
    {
        return return_type(source);
    }

    // conversion is checked dynamically, and could throw
    template <typename srcT, int srcMin, int srcMax>
    static return_type coerce(BoundedValue<srcT, srcMin, srcMax> const& source)
    {
        return return_type(static_cast<srcT>(source));
    }
};

template <typename dstT, int dstMin, int dstMax,
          typename srcT, int srcMin, int srcMax>
auto safe_bounded_cast(BoundedValue<srcT, srcMin, srcMax> const& source)
    -> BoundedValue<dstT, dstMin, dstMax>
{
    return BoundedCastHelper<dstT, dstMin, dstMax>::convert(source);
}

template <typename dstT, int dstMin, int dstMax,
          typename srcT, int srcMin, int srcMax>
auto unsafe_bounded_cast(BoundedValue<srcT, srcMin, srcMax> const& source)
    -> BoundedValue<dstT, dstMin, dstMax>
{
    return BoundedCastHelper<dstT, dstMin, dstMax>::coerce(source);
}