Tips For Resolving The L-value Error

Over the past few days, some of our readers have informed us that they have encountered a 1 value error.

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This error occurs when we put constants to the left of the = operator and variables to the right. So they take arr as the address and the stop side is constant, so the error will most likely appear as the required L value.

ArchnzacIt is used as a global variable and indicates a useful error that occurred during a function call. You can find various error codes in header file. This allows a good C programmer to check the return values ​​and, based on the return value, take appropriate action.Arkansas

“lvalue” is an assignment target. “L” stands for left, since the left side is a symbol. Rvalue is the correct hand value and generates a value that cannot and cannot be assigned directly. If you get an “lvalue required”, that person has an expression that returns an rvalue when an lvalue is required.

For the model, a constant is an r-value, but not an l-value of an. So:

  1 = 2; // Not well trained, rvalue assignmentint i; (i + 1) is 2; // Incorrectly formed, rvalue gets bolgreat value. 

  int i;i = 2; 

Do it. Note that you can return an lvalue from a function; For example, you can get a reference to an object that uses the = () operator.

As Pavel Minaev pointed out in the comments, this is not the usual definition of lvalues ​​and rvalues ​​in the respective language, but rather an attempt to inform someone who has a bug in using rvalues ​​and where an lvalue is considered a necessary criterion. C ++ is a language because of its many details; If you want to come officially, you must contact an authorized representative.

The terms lvalue and rvalue are not commonly found in C / C ++ programming, but when you do, their meaning is usually not immediately apparent. Most often, these terms are included in compiler error and warning messages. Compilation example if you follow with gcc :

That’s right, this code is inherently kinky and not what you would write, but as the lvalue error message mentions, it is a term often found in C / C ++ tutorials. Another example is compiling this Password with g ++ :

Here the error is again related to the cryptic rvalue. So what does lvalue rvalue mean in C and C ++? This is what I want to explore in this article.

 int foo () return 2;int main ()    foo () is 2;    Returns 0;

 test.c: In the "main" aspect:test.c: 8: 5: lvalue error: required as an accessible assignment operand

 testcpp.cpp: in int & foo () function:testcpp.cpp: 5:12: Error: Incorrect initialization of a non-persistent linkThe type to do with 'int &' of type 'int'

Practical Definition

This section contains an intentionally simplified explanation of lvalues ​​and rvalues. The rest of the article will explain this.

The

lvalue (locator value) represents the object in which the fact occupies an identifiable location (that is, memory generates an address).

rvalue is defined by omission, saying that each expression is either an lvalue or an rvalue. Thus, a large rvalue, far from the definition of an lvalue above, is a term that does not mean that an object occupies an identifiable location that carries memory.

Simple Examples

The above terms may seem vague, so throughit is extremely important to properly omit a few simple examples.

Suppose we have a perceived and assigned integer variable:

The assignment expects a lovely lvalue as its left operand, and var is likely to be an lvalue, since it is an identifiable element. The following elements are not valid on your current page:

Neither the integer constant 4 nor the expression var + 1 are lvalues ​​(which makes them rvalues). This is not an lvalue, because both answers are temporary expressions without an exact identifiable location (ie they may have recently been in the temporary register for the most important part of the computation). there is no semantic meaning – there is nowhere to assign.

Thus, the meaning of the error message in the first code snippet should be clear. foo tax returns a temporary value, which is a very good rvalue. Attempting to assign it is an effective error, if so, foo () implies 2; the compiler complains that it expected you to see an lvalue with the assignment operator on the lefta.

However, not all mappings to generic function call results are invalid. For example, the C ++ references make this possible:

Here foo gets a link, which is a useful value I to which it can be assigned. In fact, C ++ ‘s ability to accept l values ​​from functions is important for implementing other overloaded operators. A typical example is overloading the hook operator [] in classes that implement some kind of search access. std :: map does the following:

The assignment of mymap [10] works because its non-const overload std :: map :: operator [] returns a map that can be assigned.

 4 implies var; // GUILT!(var + 1) = 4; // GUILT!

 int globalvar = 20;int & foo ()    Pay for a global variable;int main ()    foo () is 10;   king 0;

 std :: map  mymap;mymap [10] = 5.6;

Editable Values ​​I.

Originally, after lvalues ​​were set for It c, this could literally mean “suitable values ​​for the left side of the assignment.” However, later, when ISO C contained the const keyword, this definition had toclarify. After all:

So, one more clarification as a way of adding. Not all lvalues ​​can remain affected. These values ​​can be changed. Formally, the C99 standard becomes mutable values, for example:

the current lvalue does not have an array type, does not use an incomplete type, does not have a last qualified const type and, if it is a complete structure or union, n ‘does not have a defined member (including, recursively, every element or part of all contained aggregates or unions) with a qualified const.

 const int means 10; // 'a' is the value 1a is equal to 10; // but cannot be assigned!

Converting Between Lvalues ​​and Rvalues ​​

Typically, verbal language constructs that operate on objects assume that rvalues ​​are required as arguments. For example, the binary addition operator '+' takes two available rvalues ​​as arguments and returns one rvalue:

As we saw earlier, a and b are two L values. Therefore, in the third line, they implicitly convert the l value to the r value. All lvalues, which, in turn, are not are arrays, functions, or incomplete types, and can be converted to rvalues.

And vice versa? Can rvalues ​​be changed to lvalues? Of course not! This would probably violate the nature of a good lvalue as defined [1].

This does not mean that lvalues ​​cannot be generated explicitly from rvalues. For example, any unary operator '*' (dereference) takes a nice new rvalue argument, but essentially yields an lvalue result. Take a look at this valid code:

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