assignment-operator

问题 Consider the following 2 programs prog1 and prog2 .Here if I try to change the value of the const qualified variable i using a pointer ptr ,I get the warning( not error) "initialization discards qualifiers from pointer target type|" ,but the program runs nevertheless and displays the new value.But if I try to change the value of i in the second program using an assignment statement,I get the error (not warning) assignment of read-only variable 'i'| . Here are confusions arising from this

问题 I know I can use the comma operator like this for (int i = 1, j = 15; j>10; i++, j--) { // do something neat } but some articles seem to suggest that the comma operator can be used outside of the for loop declaration, for example int j = 2, k = 4 ; int x ; // Assignment statement with comma operator x = j + 1, k ; source: http://www.cs.umd.edu/~clin/MoreJava/ControlFlow/comma.html or int x = (expression) ? (i++,2) : 3; source: https://stackoverflow.com/a/12047433/1084813 This would be a neat

问题 The assignment operator in base class does not seem to be available in derived class. Given this code: #include <iostream> class A{ int value; public: A& operator=(int value){ this->value = value; return *this; } }; class B : public A{}; int main(){ B b; b = 0; // Does not work return 0; } GCC 6.4 says: error: no match for 'operator=' (operand types are 'B' and 'int') What is happening? 回答1: Every class has at least one assignment operator implicitly defined when we don't provide one

问题 From the boost library documentation I read this: Conceptually, smart pointers are seen as owning the object pointed to, and thus responsible for deletion of the object when it is no longer needed. I have a very simple problem: I want to use RAII for pointer attributes of a class that is Copyable and Assignable. The copy and assignment operations should be deep: every object should have its own copy of the actual data. Also, RTTI needs to be available for the attributes (their type may also

问题 From the boost library documentation I read this: Conceptually, smart pointers are seen as owning the object pointed to, and thus responsible for deletion of the object when it is no longer needed. I have a very simple problem: I want to use RAII for pointer attributes of a class that is Copyable and Assignable. The copy and assignment operations should be deep: every object should have its own copy of the actual data. Also, RTTI needs to be available for the attributes (their type may also

问题 While playing with implementing a virtual assignment operator I have ended with a funny behavior. It is not a compiler glitch, since g++ 4.1, 4.3 and VS 2005 share the same behavior. Basically, the virtual operator= behaves differently than any other virtual function with respect to the code that is actually being executed. struct Base { virtual Base& f( Base const & ) { std::cout << "Base::f(Base const &)" << std::endl; return *this; } virtual Base& operator=( Base const & ) { std::cout <<

问题 I want to define (<- and access the name of the left hand side argument : *<- functions use internally an intermediate '*tmp*' variable. Is it still possible to get the name of x ? `(<-` <- function(x,value){ print(deparse(substitute(value))) print(deparse(substitute(x))) print(match.call()) value } foo <- 0 (foo) <- 3 # [1] "3" # [1] "*tmp*" # `(<-`(x = `*tmp*`, value = 3)# [1] "3" I want to get "foo" from inside the function. I tried to hack it by using tracemem , i.e. calling sapply(ls

问题 This question already has answers here : problems with char array = char array (2 answers) Closed 4 years ago . Well here is my first post. I've been trying to do this choice choosing thing and I want the user to choose only numbers instead of typing them down (easier) but when I want the numbers to equal a string, it says "array type char[30] is not assignable". Even if at the back I put semi-colon or not. #include <stdio.h> int main() { int choice1; char word[30]; printf("You have three

问题 I know in some languages the following: a += b is more efficient than: a = a + b because it removes the need for creating a temporary variable. Is this the case in C? Is it more efficient to use += (and, therefore also -= *= etc) 回答1: So here's a definitive answer... $ cat junk1.c #include <stdio.h> int main() { long a, s = 0; for (a = 0; a < 1000000000; a++) { s = s + a * a; } printf("Final sum: %ld\n", s); } michael@isolde:~/junk$ cat junk2.c #include <stdio.h> int main() { long a, s = 0;

问题 The code is as follows: #include <iostream> using namespace std; class A { }; A rtByValue() { return A(); } void passByRef(A &aRef) { // do nothing } int main() { A aa; rtByValue() = aa; // compile without errors passByRef(rtByValue()); // compile with error return 0; } The g++ compiler gives the following error: d.cpp: In function ‘int main()’: d.cpp:19:23: error: invalid initialization of non-const reference of type ‘A&’ from an rvalue of type ‘A’ d.cpp:12:6: error: in passing argument 1 of