Error Lvalue Required
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What Does Lvalue In C Means
with us Stack Overflow Questions Jobs Documentation Tags Users Badges Ask Question x Dismiss Join the Stack Overflow Community Stack Overflow r value error in c is a community of 4.7 million programmers, just like you, helping each other. Join them; it only takes a minute: Sign up “l-value required” error up vote 1 down vote favorite 2 When do we
Rvalue In C
get "l-value required" error...while compiling C++ program???(i am using VC++ ) c++ lvalue rvalue share|improve this question edited Aug 30 '09 at 8:06 Brandon E Taylor 15.2k53256 asked Aug 30 '09 at 7:53 JAY G 1642618 8 Please post a code snippet. –Filip Navara Aug 30 '09 at 7:55 3 You should definitely extend the question with at least a copy-pasted build error message, since people often what is l value and r value in c feed those directly to a search engine. –sharkin Aug 30 '09 at 8:23 add a comment| 8 Answers 8 active oldest votes up vote 9 down vote accepted An "lvalue" is a value that can be the target of an assignment. The "l" stands for "left", as in the left hand side of the equals sign. An rvalue is the right hand value and produces a value, and cannot be assigned to directly. If you are getting "lvalue required" you have an expression that produces an rvalue when an lvalue is required. For example, a constant is an rvalue but not an lvalue. So: 1 = 2; // Not well formed, assigning to an rvalue int i; (i + 1) = 2; // Not well formed, assigning to an rvalue. doesn't work, but: int i; i = 2; Does. Note that you can return an lvalue from a function; for example, you can return a reference to an object that provides a operator=(). As pointed out by Pavel Minaev in comments, this is not a formal definition of lvalues and rvalues in the language, but attempts to give a description to someone confused about an error about using an rvalue where an lvalue is required. C++
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Lvalue Required In Function Main
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Lvalue Required Error In C++
x Dismiss Join the Stack Overflow Community Stack Overflow is a community of 4.7 million programmers, just like you, helping each other. Join them; lvalue required error in turbo c++ 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 185k33248302 asked http://stackoverflow.com/questions/1353384/l-value-required-error 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 an assignment where http://stackoverflow.com/questions/3896637/lvalue-required 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 answered Oct 9 '10 at 8:55
here for a quick overview of the site Help Center Detailed answers to any questions you might have Meta Discuss the workings http://stackoverflow.com/questions/6162438/lvalue-required-as-left-operand-of-assignment and policies of this site About Us Learn more about Stack Overflow the company Business Learn more about hiring developers or posting ads with us Stack Overflow Questions Jobs Documentation Tags http://eli.thegreenplace.net/2011/12/15/understanding-lvalues-and-rvalues-in-c-and-c Users Badges Ask Question x Dismiss Join the Stack Overflow Community Stack Overflow is a community of 4.7 million programmers, just like you, helping each other. Join them; it only in c takes a minute: Sign up lvalue required as left operand of assignment up vote 16 down vote favorite 4 Why am I getting lvalue required as left operand of assignment with a single string comparison? How can I fix this in C? if (strcmp("hello", "hello") = 0) Thanks! c lvalue strcmp share|improve this question edited May 28 '11 at 15:08 value in c Kiril Kirov 23.6k875137 asked May 28 '11 at 15:06 Joseph 134239 possible duplicate of "lvalue required as left operand of assignment " error –Bo Persson May 28 '11 at 15:22 add a comment| 4 Answers 4 active oldest votes up vote 30 down vote You need to compare, not assign: if (strcmp("hello", "hello") == 0) ^ Because you want to check if the result of strcmp("hello", "hello") equals to 0. About the error: lvalue required as left operand of assignment lvalue means an assignable value (variable), and in assignment the left value to the = has to be lvalue (pretty clear). Both function results and constants are not assignable (rvalues), so they are rvalues. so the order doesn't matter and if you forget to use == you will get this error. (edit:)I consider it a good practice in comparison to put the constant in the left size, so if you write = instead of ==, you will get a compilation error. for example: int a = 5; if (a = 0) // Always evaluated as false, no error
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 o