Lvalue Error
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Lvalue Required Error In Turbo C++
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R Value Required Error In C
in a C compiler error? [closed] up vote 0 down vote favorite #include
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R Value Error In C
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Lvalue Required Char Array
Question x Dismiss Join the Stack Overflow Community Stack Overflow is a community of 6.2 million programmers, just like you, helping each other. Join how to remove lvalue required error in c++ them; it only takes a minute: Sign up lvalue required up vote 7 down vote favorite 1 what does the error message "Lvalue required" actually mean? c lvalue share|improve this question edited Oct 9 '10 at 8:58 Paul Dixon http://stackoverflow.com/questions/13524104/what-does-lvalue-required-mean-in-a-c-compiler-error 185k33248302 asked Oct 9 '10 at 8:48 alfesani 132229 possible duplicate of "l-value required" error –Paul Dixon Oct 9 '10 at 9:05 @Paul that's about C++ though. –Johannes Schaub - litb Oct 9 '10 at 9:08 add a comment| 5 Answers 5 active oldest votes up vote 6 down vote An lvalue is something that can appear on the left side of an assignment, in other words 'something that can be assigned' So, look for http://stackoverflow.com/questions/3896637/lvalue-required an assignment where the left hand side isn't 'assignable', for example, something as simple as this might trigger such an error if (0 = foo) { } Here you've got an attempt to assign to a constant because of accidentally using = rather than == See also often used seldom defined terms: lvalue lvalue and rvalue “l-value required” error share|improve this answer edited Oct 9 '10 at 9:04 answered Oct 9 '10 at 8:51 Paul Dixon 185k33248302 add a comment| up vote 5 down vote It means the implementation expects an object, but you just passed a value or function. This happens for assignments you passed a non-lvalue or for address-of operations applied to non-functions. Lvalue stands for "location value" and means an expression that refers to an object either declared as register or to a memory location. Something like 42 is a value that matches neither criteria. More formally there are three categories Lvalues: Referring to objects. This includes objects declared const. Such are non-modifiable lvalues. Function designators: Referring to functions. printf is a function designator, but &printf is not, while *&printf is again. Others: Sometimes called "rvalue" and by the Standard described as "the value of an expression". Examples are var + 0 (yielding a value not associated with objects anymore), or an enumerator of an enumeration. &printf belongs to this category. share|improve this answer edited Oct 9 '10 at 9:08 answ
operator If you've been programming in either C or C++ for a while, it's likely that you've heard the terms lvalue (pronounced "ELL-value") and rvalue (pronounced "AR-value"), if only because they occasionally appear in http://ieng9.ucsd.edu/~cs30x/Lvalues%20and%20Rvalues.htm compiler error messages. There's also a good chance that you have only a vague understanding of what they are. If so, it's not your fault. Most books on C or C++ do http://eli.thegreenplace.net/2011/12/15/understanding-lvalues-and-rvalues-in-c-and-c not explain lvalues and rvalues very well. (I looked in a dozen books and couldn't find one explanation I liked.) This may be due to of the lack of a consistent error in definition even among the language standards. The 1999 C Standard defines lvalue differently from the 1989 C Standard, and each of those definitions is different from the one in the C++ Standard. And none of the standards is clear. Given the disparity in the definitions for lvalue and rvalue among the language standards, I'm not prepared to offer precise definitions. However, I can error in c explain the underlying concepts common to the standards. As is often the case with discussions of esoteric language concepts, it's reasonable for you to ask why you should care. Admittedly, if you program only in C, you can get by without understanding what lvalues and rvalues really are. Many programmers do. But understanding lvalues and rvalues provides valuable insights into the behavior of built-in operators and the code compilers generate to execute those operators. If you program in C++, understanding the built-in operators is essential background for writing well-behaved overloaded operators. Basic conceptsKernighan and Ritchie coined the term lvalue to distinguish certain expressions from others. In The C Programming Language (Prentice-Hall, 1988), they wrote "An object is a manipulatable region of storage; an lvalue is an expression referring to an object....The name 'lvalue' comes from the assignment expression E1 = E2 in which the left operand E1 must be an lvalue expression." In other words, the left and right operands of an assignment expression are themselves expressions. For the assignment to be valid, the left operand must refer to an object-it must be an lvalue. The right operand
but when one does, it's usually not immediately clear what they mean. The most common place to run into these terms are in compiler error & warning messages. For example, compiling the following with gcc: int foo() {return 2;} int main() { foo() = 2; return 0; } You get: test.c: In function 'main': test.c:8:5: error: lvalue required as left operand of assignment True, this code is somewhat perverse and not something you'd write, but the error message mentions lvalue, which is not a term one usually finds in C/C++ tutorials. Another example is compiling this code with g++: int& foo() { return 2; } Now the error is: testcpp.cpp: In function 'int& foo()': testcpp.cpp:5:12: error: invalid initialization of non-const reference of type 'int&' from an rvalue of type 'int' Here again, the error mentions some mysterious rvalue. So what do lvalue and rvalue mean in C and C++? This is what I intend to explore in this article. A simple definition This section presents an intentionally simplified definition of lvalues and rvalues. The rest of the article will elaborate on this definition. An lvalue (locator value) represents an object that occupies some identifiable location in memory (i.e. has an address). rvalues are defined by exclusion, by saying that every expression is either an lvalue or an rvalue. Therefore, from the above definition of lvalue, an rvalue is an expression that does not represent an object occupying some identifiable location in memory. Basic examples The terms as defined above may appear vague, which is why it's important to see some simple examples right away. Let's assume we have an integer variable defined and assigned to: int var; var = 4; An assignment expects an lvalue as its left operand, and var is an lvalue, because it is an object with an identifiable memory location. On the other hand, the following are invalid: 4 = var; // ERROR! (var + 1) = 4; // ERROR! Neither the constant 4, nor the expression var + 1 are lvalues (which makes them rvalues). They're not lvalues because both are temporary results of expressions, which don't have an identifiable memory location (i.e. they can just reside in some temporary register for the duration of the computation). Therefore, assigning to them makes no semantic sense - there's nowhere to assign to. So it should now be clear what the error message in the first code snippet means. foo returns a temporary value which is an rvalue. Attempting to assign to it is an error, so when seeing foo() = 2; the compiler complains that it expected to see an lvalue on the left-hand-side of the assignment statement. Not all assignments to results of function c