From 08e4b73acdd297f84ed4f12b84d8ca106fd6a2a1 Mon Sep 17 00:00:00 2001 From: Changkun Ou Date: Sun, 7 Jun 2026 16:12:31 +0200 Subject: [PATCH] book: clarify value category is a property of expressions The lvalue/rvalue section defined value categories in terms of objects, which obscures the fundamental point that value category is a property of expressions (each expression is exactly one of lvalue/prvalue/xvalue). Add a clarifying note citing cppreference while keeping the intuitive descriptions as labeled approximations. Also fix pvalue -> prvalue typo. Fixes #275 --- book/en-us/03-runtime.md | 9 ++++++++- book/zh-cn/03-runtime.md | 2 ++ 2 files changed, 10 insertions(+), 1 deletion(-) diff --git a/book/en-us/03-runtime.md b/book/en-us/03-runtime.md index 8ecd295..f4fcffa 100644 --- a/book/en-us/03-runtime.md +++ b/book/en-us/03-runtime.md @@ -241,6 +241,13 @@ and making the function object container `std::function` possible. To understand what the rvalue reference is all about, you must have a clear understanding of the lvalue and the rvalue. +> Strictly speaking, **a value category is a property of an *expression*, not of an +> object**: every C++ expression belongs to exactly one of the three primary value +> categories — lvalue, prvalue (pure rvalue), and xvalue (expiring value). The phrasings +> below ("an lvalue is a persistent object", "an rvalue is a temporary object") are +> intuitive approximations meant to help beginners; for the rigorous definitions, see +> [cppreference: value category](https://en.cppreference.com/w/cpp/language/value_category). + **lvalue, left value**, as the name implies, is the value to the left of the assignment symbol. To be precise, an lvalue is a persistent object that still exists after an expression (not necessarily an assignment expression). @@ -252,7 +259,7 @@ In C++11, in order to introduce powerful rvalue references, the concept of rvalue values ​​is further divided into: prvalue, and xvalue. -**pvalue, pure rvalue**, purely rvalue, either purely literal, +**prvalue, pure rvalue**, purely rvalue, either purely literal, such as `10`, `true`; either the result of the evaluation is equivalent to a literal or anonymous temporary object, for example `1+2`. Temporary variables returned by non-references, temporary variables generated diff --git a/book/zh-cn/03-runtime.md b/book/zh-cn/03-runtime.md index 94517dc..3a0c01b 100644 --- a/book/zh-cn/03-runtime.md +++ b/book/zh-cn/03-runtime.md @@ -212,6 +212,8 @@ int main() { 要弄明白右值引用到底是怎么一回事，必须要对左值和右值做一个明确的理解。 +> 严格来说，**值类别 (value category) 是表达式的性质，而不是对象的性质**：每个 C++ 表达式都恰好属于三种基本值类别之一——左值 (lvalue)、纯右值 (prvalue)、将亡值 (xvalue)。下文中“左值是持久对象”“右值是临时对象”一类的说法，是为了便于初学者建立直觉的近似描述；严谨的定义请参阅 [cppreference：值类别](https://zh.cppreference.com/w/cpp/language/value_category)。 + **左值** (lvalue, left value)，顾名思义就是赋值符号左边的值。准确来说， 左值是表达式（不一定是赋值表达式）后依然存在的持久对象。