避免带默认参数的类模板的支架 [英] Avoiding Brackets for Class Template Having Default Parameter

问题描述

我有一个类似于下面的类模板，该模板旨在包含解析CSV文件时使用的一些配置设置：

I have a class template similar to the one that follows below that is designed to contain some configuration settings used when parsing CSV files:

template <typename InputIterator = default_all>
class icsv_params
{
    // Iterator to a data structure containing the columns
    // that should be read.
    typedef InputIterator iterator;
    // This is a bitmask type.
    typedef detail::icsv_op icsv_op;

    static const icsv_op noqt = icsv_op(detail::csv_flags::noqt);
    static const icsv_op quot = icsv_op(detail::csv_flags::quot);
    static const icsv_op mmap = icsv_op(detail::csv_flags::mmap);

    // The rest of the class definition isn't relevant.
};

现在，当用户希望向数据结构提供开始和结束迭代器时，模板参数很重要包含应解析的列数;然而，如果用户无法将迭代器提供为参数，则类应自动假定应对所有列进行解析。

Now, the template parameter is important when the user wishes to supply start and end iterators to a data structure containing the numbers of the columns that should be parsed; however, should the user fail to provide the iterators as parameters, the class should automatically assume that all of the columns should be parsed.

在第二种情况下，代码声明一个类的实例看起来很笨重：

In the second case, the code to declare an instance of the class looks unwieldy:

icsv_params<> params(...);

此外，位掩码类型 noqt ， 和 mmap 仅由此类使用，因此将它们放在类定义中是有意义的;然而，如果用户希望使用这些位掩码类型，那么这样做的代码也是笨拙的：

Additionally, the bitmask types noqt, quot, and mmap are used by this class only, so it makes sense to put them inside the class definition; however, should the user wish to use these bitmask types, the code to do so is also unwieldy:

icsv_params<> params(icsv_params<>::noqt);

如何使用它，以便用户不需要提供尖括号来表示缺席的模板参数？如果没有办法这样做，你会建议什么替代方案？

How can I make it so that the user does not need to provide the angle brackets to indicate the absence of a template parameter? If there is no way to do so, what alternative would you suggest?

推荐答案

不幸的是，这是C ++语法。 IIRC，在C ++ 0x中，有关联的命名空间（解决你的第二个问题）。

Unfortunately this is C++ syntax. IIRC, in C++0x, there are associated namespaces (which solve your second question).

对于第一个，typedef应该做，àla STL：

For the first one, a typedef should do, à la STL:

template <typename InputIterator = default_all>
class basic_icsv_params
{
    ...
};

typedef basic_icsv_params<> icsv_params:

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