declaration

问题 The following code is based on a snippet from here. I want a templated function that accepts a reference to an array of size that is deduced when the function is instantiated: template<int size> void myFunction( SomeType(&param)[size] ) { //use param and size here } //called like this: SomeType array[SomeConstant]; myFunction( array ); //SomeConstant magically gets into the function as "size" Now I'm confused with SomeType(&param)[size] . I'd expect the following to work: template<int size>

问题 I am solving a basic Java string problem on CodingBat.com (extraFront). The task is to, given a string of any length, return the two first characters repeated three times. This first example is what I ended up doing intuitively: public String extraFront(String str) { if (str.length() <= 2){ String front = str; }else{ String front = str.substring(0,2); } return front+front+front; } Which gives me an error, front can not be resolved . I guessed that I needed to define the variable outside the

问题 // In A.h class A { public: enum eMyEnum{ eOne, eTwo, eThree }; public: A(eMyEnum e); } // In B.h #include "A.h" class B { B(); private: A memberA; } // In B.cpp #include "B.h" B::B(void) : memberA(A::eOne) {} The declaration to 'memberA' gives me a compile error using the g++ compiler: error: 'A::eOne' is not a type How can I overcome this? Do I simply need to create a default constructor that takes no parameters? 回答1: It sounds like you are trying to initialise a member variable. You could

问题 The language standard says: [ Note: Clause 5 defines the syntax, order of evaluation, and meaning of expressions.58 An expression is a sequence of operators and operands that specifies a computation. An expression can result in a value and can cause side effects. — end note ] E.g. I've code below: int i=1; A obj; So, do both statements above, count as "expression"? Some people on stackoverflow says "int i=1;" is not an expression. This is quite odd to me. (1) An initialization is a kind of

问题 I tried to compile the following code using g++ (gcc version 4.8.2 (Debian 4.8.2-1)), with -Wall flag (adding the -Wextra flag does not change anything for me). #include <iostream> using namespace std ; int main() { int i ; cout << i << endl ; } It gave this warning: test.cpp: In function ‘int main()’: test.cpp:7:13: warning: ‘i’ is used uninitialized in this function [-Wuninitialized] cout << i << endl ; But the following code does not yield any warning: #include <iostream> using namespace

问题 In a project I am working on, I am currently implementing an algorithm which is templated. I had some problems organizing my function declarations and definitions, because templates are involved (I asked a question here about how to deal with non-templated functions "grouped" together with the templated ones). This, however, made me wonder about a general proper organization of such files. I wanted to separate all the definitions from all the declarations (just for readability purposes, even

问题 The following namespace definition fails to compile when the first declaration is commented out. If the first declaration of foo is uncommented, then it compiles just fine. namespace Y { //void foo(); void ::Y::foo(){} } The relevant part in the standard (§8.3¶1) says: When the declarator-id is qualified, the declaration shall refer to a previously declared member I understand that this rule prevents the introduction of names into other namespaces. I wonder if that rule could be relaxed to

问题 What is the difference between the following two Perl variable declarations? my $foo = 'bar' if 0; my $baz; $baz = 'qux' if 0; The difference is significant when these appear at the top of a loop. For example: use warnings; use strict; foreach my $n (0,1){ my $foo = 'bar' if 0; print defined $foo ? "defined\n" : "undefined\n"; $foo = 'bar'; print defined $foo ? "defined\n" : "undefined\n"; } print "==\n"; foreach my $m (0,1){ my $baz; $baz = 'qux' if 0; print defined $baz ? "defined\n" :

问题 For readability, I think the first code block below is better. But is the second code block faster? First Block: for (int i = 0; i < 5000; i++){ int number = rand() % 10000 + 1; string fizzBuzz = GetStringFromFizzBuzzLogic(number); } Second Block: int number; string fizzBuzz; for (int i = 0; i < 5000; i++){ number = rand() % 10000 + 1; fizzBuzz = GetStringFromFizzBuzzLogic(number); } Does redeclaring variables in C++ cost anything? 回答1: I benchmarked this particular code, and even WITHOUT

问题 Given # package main; our $f; sub f{} sub g {} 1; How can I determine that $f , but not $g , has been declared? Off the cuff, I'd thought that *{main::g}{SCALAR} might be undefined, but it is a bona fide SCALAR ref. Background: I'd like to import a variable into main:: , but carp or croak if that variable is already declared. EDIT Added an f subroutine in response to @DVK's initial answer. ANSWER (2010-07-27) This isn't easy, but it is possible. An eval technique is most portable, working on