In general there doesnt seem to be a way to do disable binding a temporary to a const reference. However, to give a substantiated answer, Id like to cite the C 2003 standard Binding a temporary to a non-const reference.(Use the compiler switch -Vbn to allow this.) Previous versions of the Borland C compilers allowed various forms of binding to non-const reference parameters. When the /Zc:referenceBinding option is specified, the compiler does not allow a non- const lvalue reference to bind to a temporary.For more information about conformance issues in Visual C, see Nonstandard Behavior. In this example, the variable a is a non-const lvalue reference. The compiler cannot bind a to the temporary initialized with the rvalue expression 2, and issues an error message.Pass by reference (C only). Lvalues and rvalues. How come a non-const reference cannot bind to a temporary object? Why is it not allowed to get non- const reference to a temporary object, which function getx() returns? Clearly, this is prohibited by C Standard but I am interested in the purpose of such restrictio. There wasnt an exception added for non-const references, because binding a temporary to a non-const reference was seen to most likely be a programmer error."It is clear that the temporary object is not constant in the sample above, because calls to non- constant functions are permitted. why second form of "bind temporary const reference" seem equivalent no-op? this case of most vexing parse, youre declaring function rather const reference a. you fix using c11 uniform initialization And attempting to compile this emits an error: error: non-const lvalue reference to type slicearray cannot bind to aThese are defined incorrectly in the book as non-const. Relatedc - Why does clang let me take a temporary by non-const reference in C03 mode.
Discussion in C started by kaede, Oct 2, 2003.Will this const reference initialized with getData() [temporaries] make that temporary value live for the lifetime of the reference itself? However, C deliberately specifies that binding a temporary object to a reference to const on the stack lengthens the lifetime of the temporary to the lifetime of the reference itself, and thus avoids what would otherwise be a common dangling- reference error. C storing a const to a temporary. What is this strange copy constructor error complaining about?Why does the second form of "bind temporary to const reference" seem to be equivalent to a no-op? Why is it not allowed to get non-const reference to a temporary object, which function getx() returns? Clearly, this is prohibited by C Standard but I am interested in the purpose of such restriction, not a reference to the standard. struct X . | Const reference. Hi, all!The side question: if there is no way to use bart:baz(const foo) from Lua, I guess the registration code should be nice and tell me this (with either compile-time or binding-time error) reference. c. language-lawyer.int main() auto const x P10.x std::cout << "extractn" GCC prints P() extract, indicating that the temporarys lifetime is not extended by the reference. Why not simply overload slurp? Std::string slurp(char const filename) std::ifstream in(filename) return slurp(in) . Int main(int argc, char argv) bool const usestdin argc 1 std::string const input usestdin ? slurp(std::cin) : slurp(argv) . It is a general solution with the conditional operator. That is, the argument i non-const and as a consequence I cannot bind it to a temporary.
const reference to temporary oddity. constant references with typedef and templates in c. In previous releases of Visual C, non-const references could be bound to temporary objects.Reference. Breaking Changes in the Visual C Compiler. The program passes the rvalue 2 as an argument to f(). At runtime, C creates a temporary object of type int with the value 2 and binds it to the reference i. The temporary and its referenceFor this reason, you may only bind references to const objects. The same rule applies to user-defined objects. c. const-reference binding to a temporary. Consider the following: string const name1 getname() string const name2 getname() where getname returns a string object. As is well known, with the introduction of move-semantics in C11, both statements can be efficient Binding temporaries to const references. From: Jens Auer . NewsgroupsHi, I was playing around with const references and temporary objects to make sure that I understood section 12.2. of the standard correctly. Rvalue references are implicitly converted to rvalues (more specifically, to xvalues) as one of the standard conversions (chapter 4 of the C standard)Rvalues (including xvalues) can be bound to const lvalue references so that you can pass a temporary to a function with such a parameter Konrad Rudolph Feb 2 12 at 14:40 Konrad: Are you familiar with the reference-to-const rules when binding a temporary?RecommendC: non-temporary const reference. res it locally. Will this const reference initialized with getData() [temporaries] make that temporary value live for the lifetime of the reference itself? Thanks, Kaede.comp.lang.c 49423 articles. 5 followers. Post Follow. Search This Blog. c - const-reference binding to a temporary in case, string-literal "billy" implicitly converted temporary string, , name3 binds temporary. obviously, name4 not temporary. The problem is that youre trying to bind a temporary to the reference, which C doesnt allow unless the reference is const. So you can do one of either the following Referenece binding to const temporaries. Temporaries, non-const yet r-value?! lifetime of temporaries. Question regarding Const Reference and Temporaries. Browse more C / C Questions on Bytes. In general there doesnt seem to be a way to do disable binding a temporary to a const reference. However, to give a substantiated answer, Id like to cite the C 2003 standard 4. const reference. 5. How to pass all parent variables to a called process.8. Unsafe Literals or Peeked Bind Variables ( ID 377847.1). 9. [g] undefined reference to (a constructor). 10. From C to C: A quick reference for aging programmers. "Christof Meerwald" wrote in message news:a1hu2k1hkn2 digitaldaemon.com Hi, this one was recently discussed on comp.lang. c.moderated (a non-const reference shouldnt be allowed to be bound to a temporary) In previous releases of Visual C, non-const references could be bound to temporary objects. Now, temporary objects can only be bound to const references. Because this feature of C is really important. In fact, in C03, if it werent for the fact that [code] const T[/code] can bind to an rvalue, thenWhy doesnt auto evaluate as an rvalue reference when a temporary is given in C? A non-const object can call both const and non-const function. const reference : - const references allow you to specify that the data referred to wont be changed.Once a reference is bound to refer to an object, it can not be bound to refer to another object. - For example This week, I learned that C has (yet another) special feature that allows the lifetime of a temporary (rvalue) to be lengthen on the stack if it is bound to a reference-to- const. C storing a const to a temporary. Show where temporaries are created in C.Section 12.2.5 in C03 says "A temporary bound to a reference member in a constructors ctor-initializer (12.6.2) persists until the constructor exits" So I tried following program. What is needed from the language? Binding temporaries to references. More on A. moveptr Example.Bjarne in his excellent text "The Design and Evolution of C" discusses the motivation for prohibiting the binding of an rvalue to a non-const reference in section 3.7. However, C deliberately specifies that binding a temporary object to a reference to const on the stack lengthens the lifetime of the temporary to the lifetime of the reference itself, and thus avoids what would otherwise be a common dangling- reference error. Here is a final attempt/summary: The C committee decided it doesnt make sense to modify temporaries, therefore, they disallowed binding to non-const references. May be some compiler implementation or historic issues were also involved, I dont know. Hi all, It seems that g tries to call a class copy constructor when binding to a const referencepublic: A() template operator T () const return 0 Select language ActionScript Ajax Android AngularJS Apache Configuration AppleScript ASP.NET (C) AutoHotkey Bash Brainfuck C C C CoffeeScript CSS CSS ExtrasIt is not clear to me whether the lifetime of a temporary object would be extended by binding it to a const reference in a ?: expression a non-const reference may only be bound to an lvalue.This attempts to bind a temporary to a non-const reference. This is illegal in C. MSVC allows it as a non-standard exception, but warns about it. C /.test200.cpp: In function int main(int, char): test200.cpp:54:11: error: invalid initialization of non- const reference of type A from an rvalue of type B fun(foo()) C Primer page 61 "we can initialize a reference to const from any expression that can be converted to the type of the reference.Because binding a reference to a temporary is almost surely not what the programmer intended, the language makes it illegal." In C, non-const references can bind to lvalues and const references can bind to lvalues or rvalues, but there is nothing that can bind to a non-const rvalue. Thats to protect people from changing the values of temporaries that are destroyed before their new value can be used. C and the const reference to temporary binding issue (Implementing the D language pass by value and by reference rules in C0X) 2011-01-11. Ive read many posts on const references but havent found what I was looking for. Could you please correct my assumption about the mechanism that binds tempora.Tags : c reference const lifetime temporary.
In C, non-const references can bind to lvalues and const references can bind to lvalues or rvalues, but there is nothing that can bind to a non-const rvalue. Thats to protect people from changing the values of temporaries that are destroyed before their new value can be used. I was wondering to what extend its possible to mimic the D language rules for pass by value and pass by reference rules in C.One of the key differences is that in D const references does not bind (as non const ones) to temporaries. A const reference prolongs a lifetime of a temporary object bound to it, so it is destroyed only when the reference goes out of scope. C does not give that feature to non-const references: std::string foo() return "foo" C language. Initialization.Otherwise, if the reference is either rvalue reference or lvalue reference to consta temporary bound to a reference parameter in a function call exists until the end of the full expression containing that function call: if the function returns a reference, which Normally, a temporary object lasts only until the end of the full expression in which it appears. However, C deliberately specifies that binding a temporary object to a reference to const on the stack lengthens the lifetime of the temporary to the lifetime of the reference itself.