Questions tagged [c++]
C++ is a general-purpose programming language. Use this tag for questions about/utilizing C++. Do not also tag questions with [c] unless you have a good reason. C and C++ are different languages. Use a versioned tag such as [c++11], [c++20] etc. for questions specific to a standard revision.
807,261
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Why is processing a sorted array faster than processing an unsorted array?
In this C++ code, sorting the data (before the timed region) makes the primary loop ~6x faster:
#include <algorithm>
#include <ctime>
#include <iostream>
int main()
{
// ...
- 499k
10152
votes
26
answers
1.0m
views
What is the '-->' operator in C/C++?
After reading Hidden Features and Dark Corners of C++/STL on comp.lang.c++.moderated, I was completely surprised that the following snippet compiled and worked in both Visual Studio 2008 and G++ 4.4. ...
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votes
1
answer
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The Definitive C++ Book Guide and List
This question attempts to collect the few pearls among the dozens of bad C++ books that are published every year.
Unlike many other programming languages, which are often picked up on the go from ...
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44
answers
1.2m
views
What are the differences between a pointer variable and a reference variable?
What is the difference between a pointer variable and a reference variable?
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What's the problem with "using namespace std;"?
I have heard using namespace std; is wrong, and that I should use std::cout and std::cin directly instead.
Why is this? Does it risk declaring variables that share the same name as something in the ...
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votes
83
answers
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How do I iterate over the words of a string?
How do I iterate over the words of a string composed of words separated by whitespace?
Note that I'm not interested in C string functions or that kind of character manipulation/access. I prefer ...
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27
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3125
votes
12
answers
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When should static_cast, dynamic_cast, const_cast, and reinterpret_cast be used?
What are the proper uses of:
static_cast
dynamic_cast
const_cast
reinterpret_cast
(type)value (C-style cast)
type(value) (function-style cast)
How does one decide which to use in which specific ...
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votes
30
answers
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views
What is the difference between #include <filename> and #include "filename"?
What is the difference between using angle brackets and quotes in an include directive?
#include <filename>
#include "filename"
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8
answers
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What is The Rule of Three?
What does copying an object mean?
What are the copy constructor and the copy assignment operator?
When do I need to declare them myself?
How can I prevent my objects from being copied?
- 260k
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10
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What are the basic rules and idioms for operator overloading?
Note: This question and the original answers are from 2010 and partially outdated. Most of it is still good and helpful, but the original text no longer covers everything there is to know about C++ ...
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Why are elementwise additions much faster in separate loops than in a combined loop?
Suppose a1, b1, c1, and d1 point to heap memory, and my numerical code has the following core loop.
const int n = 100000;
for (int j = 0; j < n; j++) {
a1[j] += b1[j];
c1[j] += d1[j];
}
...
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votes
5
answers
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What is the copy-and-swap idiom?
What is the copy-and-swap idiom and when should it be used? What problems does it solve? Does it change for C++11?
Related:
What are your favorite C++ Coding Style idioms: Copy-swap
Copy constructor ...
- 499k
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votes
19
answers
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Why can templates only be implemented in the header file?
Quote from The C++ standard library: a tutorial and handbook:
The only portable way of using templates at the moment is to implement them in header files by using inline functions.
Why is this?
(...
- 29.6k
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8
answers
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C++11 introduced a standardized memory model. What does it mean? And how is it going to affect C++ programming?
C++11 introduced a standardized memory model, but what exactly does that mean? And how is it going to affect C++ programming?
This article (by Gavin Clarke who quotes Herb Sutter) says that,
The ...
- 358k
2180
votes
10
answers
313k
views
Why is reading lines from stdin much slower in C++ than Python?
I wanted to compare reading lines of string input from stdin using Python and C++ and was shocked to see my C++ code run an order of magnitude slower than the equivalent Python code. Since my C++ is ...
2164
votes
14
answers
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What is a smart pointer and when should I use one?
What is a smart pointer and when should I use one?
- 96.7k
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votes
18
answers
1.2m
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What is the effect of extern "C" in C++?
What exactly does putting extern "C" into C++ code do?
For example:
extern "C" {
void foo();
}
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votes
20
answers
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views
How do I profile C++ code running on Linux?
How do I find areas of my code that run slowly in a C++ application running on Linux?
- 26.1k
2128
votes
29
answers
4.7m
views
How to convert int to string in C++?
How can I convert from int to the equivalent string in C++? I am aware of two methods. Is there another way?
(1)
int a = 10;
char *intStr = itoa(a);
string str = string(intStr);
(2)
int a = 10;
...
- 24.9k
2094
votes
12
answers
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views
What is move semantics?
I've just finished listening to the Software Engineering radio podcast interview with Scott Meyers regarding C++11. Most of the new features made sense to me, with the exception of one. I still don't ...
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votes
8
answers
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Regular cast vs. static_cast vs. dynamic_cast [duplicate]
I've been writing C and C++ code for almost twenty years, but there's one aspect of these languages that I've never really understood. I've obviously used regular casts i.e.
MyClass *m = (MyClass *)...
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votes
23
answers
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views
Image Processing: Algorithm Improvement for 'Coca-Cola Can' Recognition
One of the most interesting projects I've worked on in the past couple of years was a project about image processing. The goal was to develop a system to be able to recognize Coca-Cola 'cans' (note ...
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1896
votes
21
answers
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views
When to use virtual destructors?
I have a solid understanding of most OOP theory but the one thing that confuses me a lot is virtual destructors.
I thought that the destructor always gets called no matter what and for every object in ...
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1894
votes
24
answers
425k
views
Why should I use a pointer rather than the object itself?
I'm coming from a Java background and have started working with objects in C++. But one thing that occurred to me is that people often use pointers to objects rather than the objects themselves, for ...
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votes
23
answers
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What is the difference between const int*, const int * const, and int const *?
I always mess up how to use const int*, const int * const, and int const * correctly. Is there a set of rules defining what you can and cannot do?
I want to know all the do's and all don'ts in terms ...
ultraman
1803
votes
40
answers
973k
views
What is an undefined reference/unresolved external symbol error and how do I fix it?
What are undefined reference/unresolved external symbol errors? What are common causes, and how do I fix and prevent these errors?
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votes
15
answers
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views
Is < faster than <=?
Is if (a < 901) faster than if (a <= 900)?
Not exactly as in this simple example, but there are slight performance changes on loop complex code. I suppose this has to do something with generated ...
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votes
10
answers
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views
What is a lambda expression, and when should I use one?
What is a lambda expression in C++11? When would I use one? What class of problem do they solve that wasn't possible prior to their introduction?
A few examples, and use cases would be useful.
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7
answers
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views
Why does changing 0.1f to 0 slow down performance by 10x?
Why does this bit of code,
const float x[16] = { 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8,
1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6};
const float z[...
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votes
14
answers
265k
views
What are rvalues, lvalues, xvalues, glvalues, and prvalues?
In C++03, an expression is either an rvalue or an lvalue.
In C++11, an expression can be an:
rvalue
lvalue
xvalue
glvalue
prvalue
Two categories have become five categories.
What are these new ...
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votes
28
answers
698k
views
Why do we need virtual functions in C++?
From what I've read, virtual functions are functions in the base class that you can override in its derived classes.
But earlier, when learning about basic inheritance, I was able to override base ...
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1638
votes
11
answers
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views
Replacing a 32-bit loop counter with 64-bit introduces crazy performance deviations with _mm_popcnt_u64 on Intel CPUs
I was looking for the fastest way to popcount large arrays of data. I encountered a very weird effect: Changing the loop variable from unsigned to uint64_t made the performance drop by 50% on my PC.
...
- 39.2k
1630
votes
23
answers
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views
Compiling an application for use in highly radioactive environments
We are compiling an embedded C++ application that is deployed in a shielded device in an environment bombarded with ionizing radiation. We are using GCC and cross-compiling for ARM. When deployed, our ...
- 66.7k
1590
votes
18
answers
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views
Cycles in family tree software
I am the developer of some family tree software (written in C++ and Qt). I had no problems until one of my customers mailed me a bug report. The problem is that the customer has two children with ...
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votes
10
answers
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views
Where and why do I have to put the "template" and "typename" keywords?
In templates, where and why do I have to put typename and template on dependent names?
What exactly are dependent names anyway?
I have the following code:
template <typename T, typename Tail> // ...
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votes
27
answers
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views
When should you use a class vs a struct in C++? [duplicate]
In what scenarios is it better to use a struct vs a class in C++?
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answers
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views
What is the difference between public, private, and protected inheritance?
What is the difference between public, private, and protected inheritance in C++?
user106599
1259
votes
8
answers
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views
What is the difference between 'typedef' and 'using'?
I know that in C++11 we can now use using to write type alias, like typedefs:
typedef int MyInt;
Is, from what I understand, equivalent to:
using MyInt = int;
And that new syntax emerged from the ...
- 68.4k
1248
votes
19
answers
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views
How do I use extern to share variables between source files?
I know that global variables in C sometimes have the extern keyword. What is an extern variable? What is the declaration like? What is its scope?
This is related to sharing variables across source ...
shilpa
1221
votes
9
answers
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views
What are POD types in C++? [duplicate]
I've come across this term POD-type a few times.
What does it mean?
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7
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push_back vs emplace_back
I'm a bit confused regarding the difference between push_back and emplace_back.
void emplace_back(Type&& _Val);
void push_back(const Type& _Val);
void push_back(Type&& _Val);
As ...
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1171
votes
11
answers
639k
views
What is the difference between g++ and gcc?
What is the difference between g++ and gcc? Which one of them should be used for general c++ development?
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votes
21
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Can a local variable's memory be accessed outside its scope?
I have the following code.
#include <iostream>
int * foo()
{
int a = 5;
return &a;
}
int main()
{
int* p = foo();
std::cout << *p;
*p = 8;
std::cout << *...
1152
votes
15
answers
640k
views
Can I call a constructor from another constructor (do constructor chaining) in C++?
As a C# developer I'm used to running through constructors:
class Test {
public Test() {
DoSomething();
}
public Test(int count) : this() {
DoSomethingWithCount(count);
...
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votes
9
answers
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views
What is std::move(), and when should it be used?
What is it?
What does it do?
When should it be used?
Good links are appreciated.
- 23.2k
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23
answers
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views
Why can't variables be declared in a switch statement?
I've always wondered this - why can't you declare variables after a case label in a switch statement? In C++ you can declare variables pretty much anywhere (and declaring them close to first use is ...
- 77.6k
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8
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views
Do the parentheses after the type name make a difference with new?
If 'Test' is an ordinary class, is there any difference between:
Test* test = new Test;
and
Test* test = new Test();
- 11.1k
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What does T&& (double ampersand) mean in C++11?
I've been looking into some of the new features of C++11 and one I've noticed is the double ampersand in declaring variables, like T&& var.
For a start, what is this beast called? I wish ...
- 868k