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The “l” in the “lvalue” stands for something on the left side of the (=) operator. You will get this error message as a result of an expression not producing an lvalue (value on the left side of an assignment). What Causes Expression Must Be a Modifiable Lvalue

– When Should an Expression in C++ Be Modifiable?.

– Which Operator Can You Use To Find the Lvalue of a Variable?.

– What Is the Meaning of Lvalue Required as Increment Operand?.

– In C++, What Is a Universal Reference.

– What Is the Meaning of Expression Must Be a Modifiable value?.

– Example One: Triggering Expression Must Be a Modifiable Lvalue in C++.

What Causes Expression Must Be a Modifiable Lvalue.

We have since then switched to another C parser that is more restrictive on this, and besides, it is not proper ISO/ANSI C code.Īll product names are trademarks or registered trademarks of their respective owners. Many compilers permit cast expressions on the left side of an assignment, including several produced by IAR Systems. As mentioned above, you can use a unary-operator before the cast-expression to get a unary-expression, such as ' *'. A cast-expression is not part of a unary-expression. The assignment requires a unary-expression on the left side.

Unary-expression assignment-operator assignment-exression

Logical-OR-expression ? expression : conditional-expression ".Īn extract from Annex B follows below: unary-expression: The reason why it does not work is that a cast does not produce an lvalue ( ISO/ANSI 9899-1990 6.3.4 and Annex B which describes the syntax of C).įrom ISO/ANSI 9899-1990 6.3.4, cast operators, footnote 44: "A cast does not yield an lvalue. You can rewrite it to: void f (void *ptr) The following example fails: void f (void *ptr) The best way is probably to rework the types so that the cast do not appear in the first place, if possible.Īn alternative is to use a temporary variable of the desired type. There are a couple of ways to rework the code to avoid this error message. This message occurs because a cast does not produce an lvalue (that is, a value that can be used on the left side of an assignment).Īlso note that a casted expression that is used with the * operator does produce an lvalue, which is why the following is OK: void f (void * ptr) You may encounter the following message if you move code to another version of the IAR C/C++ Compiler: Error: expression must be a modifiable lvalue