如何使用Lambda避免代码重复const和非const收集处理

问题描述

限时删除！！

不适用于C ++ 17中的该模式：

The answer here does not work for this pattern in C++17:

template <typename Processor>
void Collection::ProcessCollection(Processor & processor) const
{
    for( int idx = -1 ; ++idx < m_LocalLimit ; )
    {
        if ( m_Data[ idx ] )
        {
            processor( m_Data[idx] );
        }
    }

    const int overflowSize = OverflowSize();

    for( int idx = -1 ; ++idx < overflowSize ; )
    {
        processor( (*m_Overflow)[ idx ] );
    }
}

// How to avoid this repetition for non-const version?
template <typename Processor>
void Collection::ProcessCollection(Processor & processor)
{
    for( int idx = -1 ; ++idx < m_LocalLimit ; )
    {
        if ( m_Data[ idx ] )
        {
            processor( m_Data[idx] );
        }
    }

    const int overflowSize = OverflowSize();

    for( int idx = -1 ; ++idx < overflowSize ; )
    {
        processor( (*m_Overflow)[ idx ] );
    }
}

由于传递给lambda的参数处理器为常量且不匹配其签名。

Due to the argument passed to the lambda Processor being const and not matching its signature.

推荐答案

您可以分解函数作为一个静态模板，并在两个模板中都使用它。我们可以使用模板来生成这两个函数：

You can factor out the function as a static template one and use it inside both. We can use the template to generate both of these functions:

struct Collection {
    // ...

    template<typename Processor>
    void ProcessCollection(Processor& processor) {
        ProcessCollectionImpl(*this, processor);
    }

    template<typename Processor>
    void ProcessCollection(Processor& processor) const {
        ProcessCollectionImpl(*this, processor);
    }

    template<typename T, typename Processor>
    static void ProcessCollectionImpl(T& self, Processor& processor) {
        for( int idx = -1 ; ++idx < self.m_LocalLimit ; )
        {
            if ( self.m_Data[ idx ] )
            {
                processor( self.m_Data[idx] );
            }
        }

        const int overflowSize = self.OverflowSize();

        for( int idx = -1 ; ++idx < overflowSize ; )
        {
            processor( (*self.m_Overflow)[ idx ] );
        }
    }
};

T& 将得出 Collection& 或 Collection const& ，具体取决于 * this

The T& will deduce Collection& or Collection const& depending on the constness of *this

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