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Commit 41b88c8

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book: redo string literal explanation (#237)
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‎book/en-us/03-runtime.md‎

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Original file line numberDiff line numberDiff line change
@@ -263,22 +263,34 @@ Temporary variables returned by non-references, temporary variables generated
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by operation expressions, original literals, and Lambda expressions
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are all pure rvalue values.
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Note that a string literal became rvalue in a class, and remains an lvalue in other cases (e.g., in a function):
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Note that a literal (except a string literal) is a prvalue. However, a string
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literal is an lvalue with type `const char` array. Consider the following examples:
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```cpp
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class Foo {
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const char*&& right = "this is a rvalue";
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public:
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void bar() {
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right = "still rvalue"; // the string literal is a rvalue
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}
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};
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#include <type_traits>
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int main() {
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const char* const &left = "this is an lvalue"; // the string literal is an lvalue
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// Correct. The type of "01234" is const char [6], so it is an lvalue
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const char (&left)[6] = "01234";
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// Assert success. It is a const char [6] indeed. Note that decltype(expr)
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// yields lvalue reference if expr is an lvalue and neither an unparenthesized
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// id-expression nor an unparenthesized class member access expression.
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static_assert(std::is_same<decltype("01234"), const char(&)[6]>::value, "");
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// Error. "01234" is an lvalue, which cannot be referenced by an rvalue reference
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// const char (&&right)[6] = "01234";
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}
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```
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However, an array can be implicitly converted to a corresponding pointer.The result, if not an lvalue reference, is an rvalue (xvalue if the result is an rvalue reference, prvalue otherwise):
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```cpp
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const char* p = "01234"; // Correct. "01234" is implicitly converted to const char*
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const char*&& pr = "01234"; // Correct. "01234" is implicitly converted to const char*, which is a prvalue.
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// const char*& pl = "01234"; // Error. There is no type const char* lvalue
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```
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**xvalue, expiring value** is the concept proposed by C++11 to introduce
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rvalue references (so in traditional C++, pure rvalue and rvalue are the same concepts),
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a value that is destroyed but can be moved.

‎book/zh-cn/03-runtime.md‎

Lines changed: 19 additions & 9 deletions
Original file line numberDiff line numberDiff line change
@@ -224,22 +224,32 @@ int main() {
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要么是求值结果相当于字面量或匿名临时对象,例如 `1+2`。非引用返回的临时变量、运算表达式产生的临时变量、
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原始字面量、Lambda 表达式都属于纯右值。
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需要注意的是,字符串字面量只有在类中才是右值,当其位于普通函数中是左值。例如:
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需要注意的是,字面量除了字符串字面量以外,均为纯右值。而字符串字面量是一个左值,类型为 `const char` 数组。例如:
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```cpp
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class Foo {
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const char*&& right = "this is a rvalue"; // 此处字符串字面量为右值
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public:
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void bar() {
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right = "still rvalue"; // 此处字符串字面量为右值
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}
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};
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#include <type_traits>
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int main() {
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const char* const &left = "this is an lvalue"; // 此处字符串字面量为左值
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// 正确,"01234" 类型为 const char [6],因此是左值
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const char (&left)[6] = "01234";
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// 断言正确,确实是 const char [6] 类型,注意 decltype(expr) 在 expr 是左值
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// 且非无括号包裹的 id 表达式与类成员表达式时,会返回左值引用
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static_assert(std::is_same<decltype("01234"), const char(&)[6]>::value, "");
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// 错误,"01234" 是左值,不可被右值引用
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// const char (&&right)[6] = "01234";
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}
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```
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但是注意,数组可以被隐式转换成相对应的指针类型,而转换表达式的结果(如果不是左值引用)则一定是个右值(右值引用为将亡值,否则为纯右值)。例如:
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```cpp
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const char* p = "01234"; // 正确,"01234" 被隐式转换为 const char*
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const char*&& pr = "01234"; // 正确,"01234" 被隐式转换为 const char*,该转换的结果是纯右值
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// const char*& pl = "01234"; // 错误,此处不存在 const char* 类型的左值
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```
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**将亡值(xvalue, expiring value)**,是 C++11 为了引入右值引用而提出的概念(因此在传统 C++ 中,
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纯右值和右值是同一个概念),也就是即将被销毁、却能够被移动的值。
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