I'm trying to use CppUnit to test a method that should execute some code only the first time it is called.
class CElementParseInputTests: public CppUnit::TestFixture {
private:
CElement* element;
public:
void setUp() {
element = new CElement();
}
void tearDown() {
delete element;
}
void test1() {
unsigned int parsePosition = 0;
CPPUNIT_ASSERT_EQUAL(false, element->parseInput("fäil", parsePosition));
}
void test2() {
unsigned int parsePosition = 0;
CPPUNIT_ASSERT_EQUAL(false, element->parseInput("pass", parsePosition));
}
The recursive method I want to test:
bool CElement::parseInput(const std::string& input, unsigned int& parsePosition) {
static bool checkedForNonASCII = false;
if(!checkedForNonASCII) {
std::cout << "this should be printed once for every test case" << std::endl;
[...]
checkedForNonASCII = true;
}
[...]
parseInput(input, parsePosition+1)
[...]
}
Since the object is recreated and then destroyed for every test case, I would expect that the string "this should be printed once for every test case" would be printed twice when running the tests, but it's only printed once. What have i missed?
That's what static local variables supposed to do.
Variables declared at block scope with the specifier static have static storage duration but are initialized the first time control passes through their declaration (unless their initialization is zero- or constant-initialization, which can be performed before the block is first entered). On all further calls, the declaration is skipped.
That means checkedForNonASCII will be initialized to false only once for the 1st call. For further calls the initialization is skipped; i.e. checkedForNonASCII won't be initialized to false again.
What the other answer said. But this is probably what you really want:
bool CElement::parseInput(const std::string& input, unsigned int& parsePosition)
{
[...] // your code for validating ascii only characters goes here
if (hasNonAsciiCharacters) {
return false;
}
return parseInputInteral(input, parsePosition);
}
bool CElement::parseInputInternal(const std::string& input, unsigned int& parsePosition)
{
[...]
parseInputInternal(input, parsePosition+1);
[...]
return result;
}
Related
I would like to create something similar to rust unsafe scope in C++.
The idea is that I have some functions performing number of checks. For example:
void check() {
if (...)
throw exception(...);
}
void foo() {
check();
// do some work
}
Now, I want to be able to call function foo() with or (in different context) without performing those checks. Ideally it would look like this:
foo(); // call foo and perform checks
unsafe {
foo(); // call foo without checks
}
My question is, is it possible to achieve something like this in compile time? Is it possible to somehow check (or act differently) from check function in what scope it is called?
I came up only with a runtime solution: to wrap it in some lambda:
unsafe([&] {
foo();
});
where unsafe is implemented as follows:
void unsafe(std::function<void()> f)
{
thread_local_flag = unsafe;
f();
thread_local_flag = safe;
}
check() function would just check for the thread_local flag and perform checks only when it is set to safe.
🤔
namespace detail_unsafe {
thread_local int current_depth;
struct unsafe_guard {
unsafe_guard() { ++current_depth; }
~unsafe_guard() { --current_depth; }
unsafe_guard(unsafe_guard const &) = delete;
unsafe_guard &operator = (unsafe_guard const &) = delete;
};
}
#define unsafe \
if(::detail_unsafe::unsafe_guard _ug; false) {} else
bool currently_unsafe() {
return detail_unsafe::current_depth > 0;
}
See it live on Coliru. Also, please don't actually define unsafe as a macro...
is it possible to achieve something like this in compile time?
Not the way you presented. Making foo a template function might give you equivalent results, though:
enum class CallType // find a better name yourself...
{
SAFE,
UNSAFE,
};
template <CallType Type = CallType::SAFE>
void foo()
{
if constexpr(Type != CallType::UNSAFE)
{
if (...)
throw ...;
}
// do some work
}
You might call it like:
foo();
foo<CallType::UNSAFE>();
Disliking templates?
Simple approach (thanks, #VTT):
void check(); // no template any more
void foo_unsafe()
{
// do some work
}
inline void foo()
{
check();
foo_unsafe();
}
Or selecting via parameter (this pattern exists in standard library, too):
struct Unsafe
{
};
inline Unsafe unsafe;
void check();
void foo(Unsafe)
{
// do some work
}
inline void foo()
{
check();
foo(unsafe);
}
Edit:
Well, in the example I presented I could do that, but in general, I can call some other function bar inside unsafe which in turn calls foo. And I don't want to specialize bar and possible other methods.
Unter this constraint, the template variant might be the closest you can get to at compile time; you don't have to specialise all the functions, but you'd need to make templates from:
template <CallType Type = CallType::SAFE>
void bar()
{
// do some other work
foo<Type>(); // just call with template parameter
// yet some further work
}
I would simply use a RAII type to toggle the unsafe flag inside a scope as such:
thread_local bool unsafe_flag = false;
/// RAII Type that toggles the flag on while it's alive
/// Possibly add a reference counter so it can be used nested
struct unsafe_scope
{
constexpr unsafe_scope() { unsafe_flag = true; }
~unsafe_scope() { unsafe_flag = false; }
};
/// Gets a value from a pointer
int get_value(int* ptr)
{
if ( unsafe_flag )
{
if ( ptr == nullptr ) { return 0; }
}
return *ptr;
}
int main()
{
int* x = nullptr;
//return get_value(x); // Doesn't perform the check
{
unsafe_scope cur_scope;
return get_value(x); // Performs the check
}
}
In order to make it nested I would add a reference counter like this:
/// RAII Type that toggles the flag on while it's alive
struct unsafe_scope
{
thread_local static size_t ref_count;
constexpr unsafe_scope()
{
unsafe_flag = true;
ref_count++;
}
~unsafe_scope()
{
ref_count--;
if ( ref_count == 0 ) { unsafe_flag = false; }
}
};
/// In source file
thread_local size_t unsafe_scope::ref_count = 0;
The ref_count doesn't need to be atomic since it's thread_local
Now I don't think there's a way to achieve the syntax you wanted with the unsafe before the scope, but if you put it right after the scope as such it should be about the same:
{ unsafe_scope cur_scope;
return get_value(x); // Performs the check
}
Edit:
I've now noticed Quentin's answer is also a RAII type, just with slightly different semantics, instead of having a global thread_local flag a function just returns if the reference counter is bigger than 0. Also the macro achieves the exact syntax you wanted, although it's also possible with this unsafe_scope by modifying his macro like this:
#define unsafe\
if (unsafe_scope cur_scope; false) {} else
His method uses C++17's if initializer, which lets you initiates a variable in the if statement, but the variable is still initialized in the else block, so it only gets destroyed after the else scope if over.
I have an application which has several functions in it. Each function can be called many times based on user input. However I need to execute a small segment of the code within a function only once, initially when the application is launched. When this same function is called again at a later point of time, this particular piece of code must not be executed. The code is in VC++. Please tell me the most efficient way of handling this.
Compact version using lambda function:
void foo()
{
static bool once = [](){
cout << "once" << endl;
return true;
} ();
cout << "foo" << endl;
}
Code within lambda function is executed only once, when the static variable is initialized to the return value of lambda function. It should be thread-safe as long as your compiler support thread-safe static initialization.
Using C++11 -- use the std::call_once
#include <mutex>
std::once_flag onceFlag;
{
....
std::call_once ( onceFlag, [ ]{ /* my code body here runs only once */ } );
....
}
Use global static objects with constructors (which are called before main)? Or just inside a routine
static bool initialized;
if (!initialized) {
initialized = true;
// do the initialization part
}
There are very few cases when this is not fast enough!
addenda
In multithreaded context this might not be enough:
You may also be interested in pthread_once or constructor function __attribute__ of GCC.
With C++11, you may want std::call_once.
You may want to use <atomic> and perhaps declare static volatile std::atomic_bool initialized; (but you need to be careful) if your function can be called from several threads.
But these might not be available on your system; they are available on Linux!
You can use local static variable:
void foo()
{
static bool wasExecuted = false;
if (wasExecuted)
return;
wasExecuted = true;
...
}
Additionally to #Basile's answer, you can use a lambda to encapsulate the static variable as follows:
if ([] {
static bool is_first_time = true;
auto was_first_time = is_first_time;
is_first_time = false;
return was_first_time; } ())
{
// do the initialization part
}
This makes it easy to convert into a general-purpose macro:
#define FIRST_TIME_HERE ([] { \
static bool is_first_time = true; \
auto was_first_time = is_first_time; \
is_first_time = false; \
return was_first_time; } ())
Which can be placed anywhere you want call-by-need:
if (FIRST_TIME_HERE) {
// do the initialization part
}
And for good measure, atomics shorten the expression and make it thread-safe:
#include <atomic>
#define FIRST_TIME_HERE ([] { \
static std::atomic<bool> first_time(true); \
return first_time.exchange(false); } ())
could you do this
have a function that return a bool or some datatype called init
I made it happen this way, you need static bool to make it happens
bool init()
{
cout << "Once " <<endl;
return true||false;// value isn't matter
}
void functionCall()
{
static bool somebool = init(); // this line get executed once
cout << "process " <<endl;
}
int main(int argc, char *argv[])
{
functionCall();
functionCall();
functionCall();
return EXIT_SUCCESS;
}
for C
#include <stdio.h>
void init()
{
printf("init\n");
}
void process()
{
static int someint = 0;
if(someint == 0)
{
someint = 1;
init();
}
printf("process\n");
}
int main()
{
process();
process();
process();
return 0;
}
std::call_once() et al. may be overkill if you don't need a totally thread-safe solution.
If not, we can make this look especially elegant when using C++17's initialisation-within-if and std::exchange():
#include <utility>
void
do_something_expensive_once()
{
if ( static auto called = false; !std::exchange(called, true) ) {
do_something_expensive();
}
}
If this is a pattern you use a lot, then we can encapsulate it via a tag type:
#include <iostream>
#include <utility>
template <typename T>
auto
call_once()
{
static auto called = false;
return !std::exchange(called, true);
}
void
do_something_expensive()
{
std::cout << "something expensive\n";
}
void
do_something_expensive_once()
{
if ( call_once<struct TagForSomethingExpensive>() ) {
do_something_expensive();
}
}
auto
main() -> int
{
for (auto i = 0; i < 5; ++i) {
do_something_expensive_once();
}
return 0;
}
This will only print something expensive a single time. Result! It also uses the ability to declare a tag struct in a template argument list, for maximal brevity.
Alternatively, you could template on a function's address, a unique integer, etc.
You can then also pass a callable to call_once(), and so on, and so forth. As usual for C++: the possibilities are endless!
With due respect to std::call_once() and the usual caveats about thread safety, here's another lightweight option which avoids unused variable warnings and keeps our flag in block scope:
for (static bool once=true; once; once=false) {
yourCodeHere();
}
Another simple solution is:
#define execute_once if(static bool b = false; b) ; else if((b = true))
Used thus:
execute_once std::cout << "Hi mum!\n";
or:
execute_once
{
std::cout << "These statements are ";
std::cout << "only executed once\n";
}
It's not thread safe, obviously. (EDIT: although just using a std::atomic_bool in place of the bool would get you there I think.)
do {
//execute code once
} while (false)
Hello guys i have static problem with my class:
class Monster
{
private:
static const bool hard;
//more staff here
};
I know that i can initiate it like const bool Monster::hard
But I want to know if i can initiate based on users input
Something like If(wantToBeHard) hard = true;
This means it must be in a method or something right?
There is any way to do this?
You can create a normal function (not a non-static member function) which asks for input from the user and returns a bool value:
bool AskUser()
{
....
}
Then simply use the return value of that function to initialize your static member.
const bool Monster::hard = AskUser();
Just to clear things up, here is a complete, compileable example:
#include <iostream>
#include <string>
bool AskUser();
class Monster
{
public:
static bool IsHard() { return hard; }
private:
static const bool hard;
};
int main()
{
if (Monster::IsHard())
std::cout << "it is hard\n";
else
std::cout << "it is not hard\n";
}
bool AskUser()
{
std::cout << "hard? ";
std::string input;
std::getline(std::cin, input);
return input.size() && input[0] == 'y';
}
const bool Monster::hard = AskUser();
If you want more control over when this initialization happens, you will have to drop your requirement for const. It shouldn't really be a problem though, as long as the member is private, you can still have complete control over whether it is changed. e.g.
class Monster
{
public:
static void SetHard()
{
static bool hard_is_set = false;
if (hard_is_set)
return;
hard_is_set = true;
hard = AskUser();
}
private:
static bool hard;
};
bool Monster::hard;
One possible problem here is that it is possible to call the SetHard function outside of main (e.g. in the initialization of another static object), if that happens, it may access the static member before it is actually created, resulting in undefined behavior. (static objects are tricky things and should be treated with care). So don't do that.
I have a constant value that never changes during run-time, but is impossible to know until run-time.
Is there a way to declare a constant (either as a member of a class or not) without defining it and also assign a computed value once (and only once) it is determined; or am I going to have to resort to a non-const declaration and use coding S & Ps (ALL_CAPS variables names, static declaration if in a class, etc.) to try and keep it from changing?
CLARIFICATION:
Though these are good answers, the real-world situation I have is more complicated:
The program has a main loop that continually runs between processing and rendering; the user can set required options and once they are set they will never change until the program is restart. An "Initialize" function is set up for anything that can be determined before the main loop, but values that are dependent on user interaction must be performed in the middle of the loop during the processing phase. (At the moment, persistent data storage techniques come to mind...)
Something like this?
const int x = calcConstant();
If it's a class member, then use the constructor initialisation list, as in Yuushi's answer.
You can define it in a struct or class and utilize an initialisation list:
#include <iostream>
struct has_const_member
{
const int x;
has_const_member(int x_)
: x(x_)
{ }
};
int main()
{
int foo = 0;
std::cin >> foo;
has_const_member h(foo);
std::cout << h.x << "\n";
return 0;
}
As a static or function-local variable:
const int x = calcConstant();
As a class member:
struct ConstContainer {
ConstContainer(int x) : x(x) {}
const int x;
};
Yes, you can make a private static singleton field with an initialization method and a gettor method. Here's an example of how to do it:
// In foo.h
class Foo
{
public:
// Caller must ensure that initializeGlobalValue
// was already called.
static int getGlobalValue() {
if (!initialized) {
... handle the error ...
}
return global_value;
}
static void initializeGlobalValue(...)
private:
static bool initialized;
static int global_value;
};
// In foo.cpp
bool Foo::initialized = false;
int Foo::global_value;
void Foo::initializeGlobalValue(...) {
if (initialized) {
...handle the error...
}
global_value = ...;
initialized = true;
}
I have an application which has several functions in it. Each function can be called many times based on user input. However I need to execute a small segment of the code within a function only once, initially when the application is launched. When this same function is called again at a later point of time, this particular piece of code must not be executed. The code is in VC++. Please tell me the most efficient way of handling this.
Compact version using lambda function:
void foo()
{
static bool once = [](){
cout << "once" << endl;
return true;
} ();
cout << "foo" << endl;
}
Code within lambda function is executed only once, when the static variable is initialized to the return value of lambda function. It should be thread-safe as long as your compiler support thread-safe static initialization.
Using C++11 -- use the std::call_once
#include <mutex>
std::once_flag onceFlag;
{
....
std::call_once ( onceFlag, [ ]{ /* my code body here runs only once */ } );
....
}
Use global static objects with constructors (which are called before main)? Or just inside a routine
static bool initialized;
if (!initialized) {
initialized = true;
// do the initialization part
}
There are very few cases when this is not fast enough!
addenda
In multithreaded context this might not be enough:
You may also be interested in pthread_once or constructor function __attribute__ of GCC.
With C++11, you may want std::call_once.
You may want to use <atomic> and perhaps declare static volatile std::atomic_bool initialized; (but you need to be careful) if your function can be called from several threads.
But these might not be available on your system; they are available on Linux!
You can use local static variable:
void foo()
{
static bool wasExecuted = false;
if (wasExecuted)
return;
wasExecuted = true;
...
}
Additionally to #Basile's answer, you can use a lambda to encapsulate the static variable as follows:
if ([] {
static bool is_first_time = true;
auto was_first_time = is_first_time;
is_first_time = false;
return was_first_time; } ())
{
// do the initialization part
}
This makes it easy to convert into a general-purpose macro:
#define FIRST_TIME_HERE ([] { \
static bool is_first_time = true; \
auto was_first_time = is_first_time; \
is_first_time = false; \
return was_first_time; } ())
Which can be placed anywhere you want call-by-need:
if (FIRST_TIME_HERE) {
// do the initialization part
}
And for good measure, atomics shorten the expression and make it thread-safe:
#include <atomic>
#define FIRST_TIME_HERE ([] { \
static std::atomic<bool> first_time(true); \
return first_time.exchange(false); } ())
could you do this
have a function that return a bool or some datatype called init
I made it happen this way, you need static bool to make it happens
bool init()
{
cout << "Once " <<endl;
return true||false;// value isn't matter
}
void functionCall()
{
static bool somebool = init(); // this line get executed once
cout << "process " <<endl;
}
int main(int argc, char *argv[])
{
functionCall();
functionCall();
functionCall();
return EXIT_SUCCESS;
}
for C
#include <stdio.h>
void init()
{
printf("init\n");
}
void process()
{
static int someint = 0;
if(someint == 0)
{
someint = 1;
init();
}
printf("process\n");
}
int main()
{
process();
process();
process();
return 0;
}
std::call_once() et al. may be overkill if you don't need a totally thread-safe solution.
If not, we can make this look especially elegant when using C++17's initialisation-within-if and std::exchange():
#include <utility>
void
do_something_expensive_once()
{
if ( static auto called = false; !std::exchange(called, true) ) {
do_something_expensive();
}
}
If this is a pattern you use a lot, then we can encapsulate it via a tag type:
#include <iostream>
#include <utility>
template <typename T>
auto
call_once()
{
static auto called = false;
return !std::exchange(called, true);
}
void
do_something_expensive()
{
std::cout << "something expensive\n";
}
void
do_something_expensive_once()
{
if ( call_once<struct TagForSomethingExpensive>() ) {
do_something_expensive();
}
}
auto
main() -> int
{
for (auto i = 0; i < 5; ++i) {
do_something_expensive_once();
}
return 0;
}
This will only print something expensive a single time. Result! It also uses the ability to declare a tag struct in a template argument list, for maximal brevity.
Alternatively, you could template on a function's address, a unique integer, etc.
You can then also pass a callable to call_once(), and so on, and so forth. As usual for C++: the possibilities are endless!
With due respect to std::call_once() and the usual caveats about thread safety, here's another lightweight option which avoids unused variable warnings and keeps our flag in block scope:
for (static bool once=true; once; once=false) {
yourCodeHere();
}
Another simple solution is:
#define execute_once if(static bool b = false; b) ; else if((b = true))
Used thus:
execute_once std::cout << "Hi mum!\n";
or:
execute_once
{
std::cout << "These statements are ";
std::cout << "only executed once\n";
}
It's not thread safe, obviously. (EDIT: although just using a std::atomic_bool in place of the bool would get you there I think.)
do {
//execute code once
} while (false)