为什么结构不能直接赋值? [英] Why structs cannot be assigned directly?

问题描述

假设我有一个完全定义的结构体，标签为 MyStruct，并假设 x, y, ..., z 是其字段的允许值.为什么是

Suppose I have a fully defined struct with tag MyStruct, and suppose that x, y, ..., z are allowed values for its fields. Why is

struct MyStruct q = {x,y,..,z};

允许，但是

struct MyStruct q;
q = {x,y,...,z};

不允许?我觉得这很烦人.第二种情况，我之前已经声明了q，我需要给每个字段一个一个赋值:

is not allowed? I find this very annoying. In the second case, where I have previously declared q, I need to assign a value to each field, one by one:

q.X = x; q.Y = y; ... q.Z = z;

其中 X, Y, ..., Z 是 MyStruct 的字段.这背后有什么原因吗?

where X, Y, ..., Z are the fields of MyStruct. Is there a reason behind this?

推荐答案

您正在寻找的是复合字面量.这是在 C99 中添加到语言中的.

What you are looking for is a compound literal. This was added to the language in C99.

您的第一个案例:

struct MyStruct q = {x,y,..,z};

是特定于初始化的语法.你的第二种情况，在语言学派中不是初始化，而是赋值.赋值的右侧必须是正确类型的结构.在 C99 之前，该语言中没有用于编写结构体字面量的语法，而这正是您要尝试做的.{x,y,..,z} 看起来像一个包含表达式的块.如果有人受到启发，试图将其视为字面值，尽管语言没有这样做，但人们无法确定其类型.(在您的上下文中，您可以做出很好的猜测.)

is a syntax specific to initialization. Your second case, in the pedantics of the language is not initialization, but assignment. The right hand side of the assignment has to be a struct of the correct type. Prior to C99 there was no syntax in the language to write a struct literal, which is what you are trying to do. {x,y,..,z} looked like a block with an expression inside. If one were inspired to try to think of it as a literal value, though the language didn't, one couldn't be sure of its type. (In your context, you could make a good guess.)

为了允许这样做并解决类型问题，C99 添加了语法以便您可以编写:

To allow this and resolve the type issue, C99 added syntax so you could write:

q = (struct MyStruct){x,y,...,z};

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