从变量获取值时，boost :: get与boost :: apply_visitor [英] boost::get vs boost::apply_visitor when fetching values from a variant

问题描述

我们在生产代码中广泛使用了boost :: variant的集合。
我们从此集合中提取值的方式是

We are using collections of boost::variant extensively in our production code. The way we extract the values from this collection is

for (auto & var : vars)
{
    switch (var.which())
    {
        case 1 :
        AVal = boost::get<A>(var);
        break;
        case 2 :
        BVal = boost::get<B> (var);
        ...
     } 
}

了解有关变体的更多信息我可以看到另一种替代方法是

Reading more about variants I can see that a different alternative would be

for (auto & var : vars)
{
    switch (var.which())
    {
        case 1 :
        AVal = boost::apply_visitor(AVisitor, var);
        break;
        case 2 :
        BVal = boost::apply_visitor(BVisitor, var);
        ...
    } 
}

忽略以下事实apply_visitor提供编译时类型安全值访问，并且功能更强大，我是否应该期望上述两种方法在运行时性能方面有任何差异？

Ignoring the fact that apply_visitor provides compile time type safe value visitation and is more powerful, should I expect any difference in terms of run time performance for either of the above approaches?

推荐答案

boost :: variant 只是与您提供的最大数据类型对齐的内存块，而整数则指定当前使用的是哪种类型。还有许多允许访问逻辑的编译时宏。

boost::variant is just a block of memory that is aligned to biggest data type you provide, and an integer specifying which of these types is currently use. And a lot of compile-time macros allowing for visitation logic.

丢弃一次或两次运行时检查，以确保获取正确的类型，访问该内存位置的其他费用（重新解释为所需的类型）应该没有其他费用。

Discarding one or two run-time checks making sure right type is getting grabbed, there should be no other cost for accessing that memory location, reinterpreted as the desired type.

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