C++11：文字 7 是纯右值？

Question

I came across this article . And its example says:

int main()
{
    int i, j, *p;

    // Correct usage: the variable i is an lvalue and the literal 7 is a prvalue.
    i = 7;
...
}

I wonder why literal 7 is a prvalue.

The standard [7.2.1.1.2] says:

A prvalue is an expression whose evaluation
initializes an object or a bit-field, or computes the value of an
operand of an operator, as specified by the context in which it
appears, or an expression that has type cv void.

The expression 7 itself doesn't initializes an object nor a bit-field, nor computes the value of an operand of an operator as 7 itself doesn't have an operator.

Maybe I don't interpret the standard correctly.

Answer 1

The standard quote you used describes the properties of prvalues, but does not define them. The definitions are scattered through the standard (and gathered on cppreference ).

The definition for your situation is found in [expr.prim.literal]/1 :

A string-literal is an lvalue, a user-defined-literal has the same value category as the corresponding operator call expression described in [lex.ext], and any other literal is a prvalue

Answer 2

The expression 7 itself doesn't initializes an object nor a bit-field

Actually it does, it initializes the 7 object. See https://en.cppreference.com/w/cpp/language/value_category

prvalue

The following expressions are prvalue expressions:

• a literal (except for string literal), such as 42 , true or nullptr ; ...