Not all lvalues can be on the l.h.s. of an assignment

[VoB]

Hi everyone,

I'm trying to go a understand a bit deeper the concept of lvalue, xvalue,glvalue,rvalue,prvalue from "The C++ Programming Language, 4th edition".

It's written: "However, not every lvalue may be used on the left-hand side of an assignment; an lvalue can refer to a constant (§7.7)"

For the sentence after ";" indeed, kooking at 7.7, we see that we can write

const double& cdr{1};


But what is an example of an lvalue that can't be used on the l.h.s. of an assignment?

Looking at https://en.cppreference.com/w/cpp/language/value_category, I see that also strings literals such as "Hello, World!" and a cast expression to lvalue reference type, such as static_cast<int&>(x) are lvalues references and clearly they cannot be on the l.h.s. of an assignment. Is this what Stroustrup had in mind?

[helios]

But what is an example of an lvalue that can't be used on the l.h.s. of an assignment?

'cdr' is. cdr = 0 is an invalid expression.

a cast expression to lvalue reference type, such as static_cast<int&>(x) are lvalues references and clearly they cannot be on the l.h.s. of an assignment.

No. If the compiler accepts the cast as valid, static_cast<int &>(x) can be on the LHS of an assignment. Why couldn't it be?

[VoB]

cdr = 0 is an invalid expression.

Ah yes, thanks.

If the compiler accepts the cast as valid, static_cast<int &>(x) can be on the LHS of an assignment. Why couldn't it be?

I never tried this but yes, it makes sense and I just tried it.

For what concerns string literals, also "Hello, World!" cannot be on the l.h.s. of an assignment, and I'd like to have a check about this now:

It is an lvalue, because in practice its type is const char *, i.e. a const array of characters, but since it's const we can't modify it.
Indeed, if I write "Hello, World"=" "; I have the compiler error: error: read-only variable is not assignable

Is my argument correct?

[helios]

That's the immediate reason, yes, but the ultimate reason is that being able to assign to a literal would raise the question of what happens to subsequent uses of that literal.

std::string s = "hello";
"hello" = "goodbye";
if (s == "hello"){
    //Do we enter here or not?
}

[VoB]

Thanks @helios

Indeed that if clause would lead to some troubles... for insrance, if s=="hello" were true, then it's like if "hello"=="goodbye" is true, which wouble be really weird...