参数包扩展中的评估顺序问题

Question

我写了一个模板，它接受一个 istream& 和一个函数，应该从 istream 中提取这个函数的所有参数，用这些参数和 return 调用函数结果。一切正常，除了函数参数的评估顺序。请参阅下面的代码、更多详细信息和最后一个问题：

#include <iostream>
#include <vector>

void Foo(int i, std::string s)
{
    std::cout << "input was " << i << " and " << s << '\n';
}

template<typename T>
T Parse(std::istream &s)
{
    T res;
    
    s >> res;
    
    return res;
}

template<typename TR, typename ... TArgs>
TR Bar(std::istream &s, TR f(TArgs...) )
{
    return f(Parse<TArgs>(s)...);
}

int main()
{
    Bar(std::cin, Foo);
}

输入：

1 2

预期输出：

input was 1 and 2

实际输出：

input was 2 and 1

我知道函数参数的评估是特定于实现的，显然这里最后一个参数首先被评估并读取第一个输入。

如何修复此代码并强制对参数执行特定的求值顺序？也许在调用函数之前分别评估它们？是否有可能在不违反标准的情况下 and/or 依赖于特定的实现或编译器？

Answer 1

例如，我们可以使用一个中间值std::tuple来强制求值顺序：

template<typename TR, typename ... TArgs>
TR Bar(std::istream &s, TR f(TArgs...) )
{
    std::tuple<TArgs...> args{Parse<TArgs>(s)...};
    return std::apply(f, std::move(args));
}

与函数参数相反，花括号列表中参数的求值顺序由它们在该列表中的顺序固定，[dcl.init.list/4]:

Within the initializer-list of a braced-init-list, the initializer-clauses, including any that result from pack expansions, are evaluated in the order in which they appear. That is, every value computation and side effect associated with a given initializer-clause is sequenced before every value computation and side effect associated with any initializer-clause that follows it in the comma-separated list of the initializer-list. [ Note: This evaluation ordering holds regardless of the semantics of the initialization; for example, it applies when the elements of the initializer-list are interpreted as arguments of a constructor call, even though ordinarily there are no sequencing constraints on the arguments of a call. ]

参数包扩展中的评估顺序问题

Problem with order of evaluation in parameter pack expansion

c++

pack-expansion

parameter-pack