pass-by-const-reference

Ampersand & with const in constructor

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问题 Can some body tell me the reason why we usually put const and & with some object which is passed in the constructor for example. Book::Book(const Date &date); The confusion that i have here is that usually & sign is used in the some function because the value is passed by reference and whatever changes happen to that variable in the function should reflect afterwards. But on the other hand const says that no assignment can be done to that variable. If some body have some good idea about that

Forbid rvalue binding via constructor to member const reference

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问题 I am working on a matrix view class, of which constructor takes a matrix as a parameter and binds it to a const reference member. I would very much like to avoid binding rvalues, since they don't bind via a constructor parameter, and we end up with a dangling reference. I came up with the following (simplified code): struct Foo{}; class X { const Foo& _foo; public: X(const Foo&&) = delete; // prevents rvalue binding X(const Foo& foo): _foo(foo){} // lvalue is OK }; Foo get_Foo() { return {};

c++ passing by const reference

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问题 In the following program body cosists of a vector of pointers. Points is a struct of x,y,z coordinates and a point_id. I believe as body is passed by const reference, the following step should produce an error. BUt the program is running without any problem. Can you please explain me why is this. void readOutFile(const Body& body, int n){ .... body.bp[0]->points.push_back(Point_id(p,i)); } 回答1: Here's the issue: body.bp[0]->points.push_back(Point_id(p,i)); ^^ Indirecting through a pointer

c++ copy construct parameter passed by value

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问题 I want freeFunct to do non const stuff on its own copy of object a. Let's say that freeFunct is required to be a free function because in real code cases it takes many different parameters, calls several public functions from all of them and there is no point in making it a non-static member function of any class. Three different ways of declaring it come to my mind. I have the feeling that the third solution is the worse. Is there any difference between the first two? Is there something

When is an object sufficiently large that there is a performance gain in passing it by reference instead of by value?

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问题 As answered in this question by Charles Bailey pass by constant reference should be considered when the object type is large but what kind of object is considered large? EDIT: OK for providing more data that could be used to provide a more concrete answer I decided to add a real world problem to this description. Suppose we have an object like this: typedef struct dipl { quint16 __id; quint16 __pos; quint16 __len; } data; And we have another object like this: class bidipl { public: bidipl();

c++ passing by const reference

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In the following program body cosists of a vector of pointers. Points is a struct of x,y,z coordinates and a point_id. I believe as body is passed by const reference, the following step should produce an error. BUt the program is running without any problem. Can you please explain me why is this. void readOutFile(const Body& body, int n){ .... body.bp[0]->points.push_back(Point_id(p,i)); } Here's the issue: body.bp[0]->points.push_back(Point_id(p,i)); ^^ Indirecting through a pointer removes any constness; rather, the constness of the result is dependent on the type of the pointer. T *t; //

Which is faster? Pass by reference vs pass by value C++

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I thought that pass by reference should be faster then pass by value because the computer isn't copying data, it just points to the address of data. But, consider the following C++ code: #include <iostream> #include <cassert> #include <cmath> using namespace std; // do not use pass by reference& with this function, it will become so slow unsigned long long computePascal(const unsigned int row, const unsigned int position) { if (position == 1 || position == row) return 1L; unsigned long long left = computePascal(row-1, position-1); unsigned long long right = computePascal(row-1, position);

Which is faster? Pass by reference vs pass by value C++

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问题 I thought that pass by reference should be faster then pass by value because the computer isn't copying data, it just points to the address of data. But, consider the following C++ code: #include <iostream> #include <cassert> #include <cmath> using namespace std; // do not use pass by reference& with this function, it will become so slow unsigned long long computePascal(const unsigned int row, const unsigned int position) { if (position == 1 || position == row) return 1L; unsigned long long

const-ref when sending signals in Qt

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问题 This is a thing that I never quite got with const-ref and I really hope that someone could explain it to me. When calling a function inside of another function, I get that const-ref is the best way when passing stack objects that I don't plan to tamper with. For example: void someInnerFunction(const QString& text) { qDebug() << text; } void someFunction() { QString test = "lala"; .... someInnerFunction(test); } So far so good, I guess. But what about signals? Isn't there any risk that comes

Why is it allowed to pass R-Values by const reference but not by normal reference?

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as the title says why is it allowed to pass R-Values(literals) by constant reference but not normal reference void display(const int& a) { cout << a ; } will work if called display(5) but without the const it won't work ****** I mean how can a const reference keep pointing to an R-Value (anonymous variable) ****** For your final question: how can a const reference keep pointing to an R-Value (anonymous variable) Here is the answer . The C++ language says that a local const reference prolongs the lifetime of temporary values until the end of the containing scope, but saving you the cost of a