参数包扩展顺序 [英] Order of parameter pack expansion

问题描述

我有2个函数来读取二进制文件。

I have 2 functions to read binary file.

第一个函数从文件中读取 sizeof（T）个字节：

1st function reads sizeof(T) bytes from file:

template<typename T>
T read() { ... some IO operations ... };

第二个函数使用每个模板参数多次调用第一个：

2nd function calls first one multiple times with each template parameter:

template<typename... Ts>
std::tuple<Ts...> read_all() {
    return std::make_tuple(read<Ts>()...);
};

除第一个函数调用顺序外，一切正常。类似

Everything works fine except of 1st function call order. For something like

uint32_t a;
uint8_t b;
std::tie(a, b) = read_all<uint32_t, uint8_t>();

第一个将被称为 read< uint8_t>（）之后读取< uint32>（），它会反转传递模板参数的顺序，并以文件中的字节顺序混乱。

the first will be called read<uint8_t>() and after that read<uint32>() which reverses order of passing template parameters and messes up with order of bytes in file.

当然，我可以用反向的模板参数顺序调用 read_all 并最终得到正确的顺序，但有更明显的方法吗？

Sure, I can call read_all with reversed order of template arguments and get right order in the end, but is there a more obvious way to do so?

推荐答案

C ++没有指定评估函数参数的顺序。如果函数的表达式都使用流中的数据，则可以获得以错误顺序读取对象的行为。

C++ does not specify the order in which a function's arguments are evaluated. If the expressions to a function all consume data from a stream, you can get behavior where objects are read in the wrong order.

从左到右计算支撑的初始化列表但是，如果你尝试这样的话，你应该得到更好的结果：

Braced initializer lists are evaluated from left to right, however, so you should get better results if you try something like:

template<typename... Ts>
std::tuple<Ts...> read_all() {
    return std::tuple<Ts...>{read<Ts>()...};
}

这篇关于参数包扩展顺序的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！