I am quite new to C++ and during solving a Codewars Kata I faced following problem.
I want to define a Variable of type std::vector with the name tmp2 in a function.
Add a char via push_back and return the value.
When I set a breakpoint at return(tmp2) and debug the code (MSVS2022), the debugger shows tmp2 as empty.
If I do the same operation in a void function the debugger shows tmp with 'c' as content
Here is my code:
#include <vector>
//turn on debugger
//set a breakpoint at return(tmp2) in function test2() and inspect tmp2
void test()
{
std::vector<char> tmp;
tmp.push_back('c'); //tmp holds 'c'
}
std::vector<char> test2()
{
std::vector<char> tmp2;
tmp2.push_back('c');
return(tmp2);//tmp2 does not contain 'c'
}
int main()
{
test();
std::vector<char> x = test2();
}
Could someone please tell me, where I am wrong?
Thanks in advance.
Regs
Marcus
I have tried to clean my project.
But I cannot find out where I am wrong.
An explanation would be very helpful.
Related
I am trying to use the testing macros with my actors but I am getting a lot of segmentation faults. I believe I have narrowed down the problem to my use of custom atoms. To demonstrate the issue I modified the 'simple actor test' from here to make the adder strongly typed.
#include "caf/test/dsl.hpp"
#include "caf/test/unit_test_impl.hpp"
#include "caf/all.hpp"
namespace {
struct fixture {
caf::actor_system_config cfg;
caf::actor_system sys;
caf::scoped_actor self;
fixture() : sys(cfg), self(sys) {
// nop
}
};
using calculator_type = caf::typed_actor<caf::result<int>(int, int)>;
calculator_type::behavior_type adder() {
return {
[=](int x, int y) {
return x + y;
}
};
}
} // namespace
CAF_TEST_FIXTURE_SCOPE(actor_tests, fixture)
CAF_TEST(simple actor test) {
// Our Actor-Under-Test.
auto aut = self->spawn(adder);
self->request(aut, caf::infinite, 3, 4).receive(
[=](int r) {
CAF_CHECK(r == 7);
},
[&](caf::error& err) {
// Must not happen, stop test.
CAF_FAIL(err);
});
}
CAF_TEST_FIXTURE_SCOPE_END()
This works great. I then took it one step further to add a custom atom called "add_numbers"
#include "caf/test/dsl.hpp"
#include "caf/test/unit_test_impl.hpp"
#include "caf/all.hpp"
CAF_BEGIN_TYPE_ID_BLOCK(calc_msgs, first_custom_type_id)
CAF_ADD_ATOM(calc_msgs, add_numbers)
CAF_END_TYPE_ID_BLOCK(calc_msgs)
namespace {
struct fixture {
caf::actor_system_config cfg;
caf::actor_system sys;
caf::scoped_actor self;
fixture() : sys(cfg), self(sys) {
// nop
}
};
using calculator_type = caf::typed_actor<caf::result<int>(add_numbers, int, int)>;
calculator_type::behavior_type adder() {
return {
[=](add_numbers, int x, int y) {
return x + y;
}
};
}
} // namespace
CAF_TEST_FIXTURE_SCOPE(actor_tests, fixture)
CAF_TEST(simple actor test) {
// Our Actor-Under-Test.
auto aut = self->spawn(adder);
self->request(aut, caf::infinite, add_numbers_v, 3, 4).receive(
[=](int r) {
CAF_CHECK(r == 7);
},
[&](caf::error& err) {
// Must not happen, stop test.
CAF_FAIL(err);
});
}
CAF_TEST_FIXTURE_SCOPE_END()
This compiles fine but produces a segmentation fault at runtime. I suspect it has something to do with the fact that I am not passing calc_msgs to anything. How do I do that? Or is something else going on?
The ID block adds the compile-time meta data. But you also need to initialize some run-time state via
init_global_meta_objects<caf::id_block::calc_msgs>();
Ideally, you initialize this state before calling any other CAF function. In particular before initializing the actor system. CAF itself uses custom main functions for its test suites to do that (cf. core-test.cpp). In your case, it would look somewhat like this:
int main(int argc, char** argv) {
using namespace caf;
init_global_meta_objects<id_block::calc_msgs>();
core::init_global_meta_objects();
return test::main(argc, argv);
}
This probably means that you would need to put the type ID block into a header file. This is nothing special about the unit tests, though. If you run a regular CAF application, you need to initialize the global meta objects as well. CAF_MAIN can do that for you as long as you pass it the type ID block(s) or you need to call the functions by hand. The CAF manual covers this in a bit more detail here: https://actor-framework.readthedocs.io/en/0.18.5/ConfiguringActorApplications.html#configuring-actor-applications.
If this is your only test at the moment, you can define CAF_TEST_NO_MAIN before including caf/test/unit_test_impl.hpp and then add the custom main function. Once you have multiple test suites, it makes sense to move the main to its own file, though.
I'm making a function to handle a message, then print the message using Serial.println(). I have it working, but ran into an issue I can't explain. The first sample code below works, the second (swapping the order of my function declaration) will compile and load, but causes the Teensy 4.1 to crash. I'm using PlatformIO on VSCode.
Can anyone tell me what is wrong with the second code, and why it will compile without error, but not run?
This works:
#include <Arduino.h>
void LogMsg(const char *msg){
Serial.println(msg);
}
void LogMsg(String s){ LogMsg(s.c_str()); }
void setup() {
// put your setup code here, to run once:
Serial.begin(115200);
Serial.println("reset");
}
void loop() {
// put your main code here, to run repeatedly:
String str3 = "testing string cat ";
uint32_t var = 12345;
LogMsg(str3 + var);
delay(500);
}
This compiles, loads, but crashes, causing continuous resets:
#include <Arduino.h>
void LogMsg(String s){ LogMsg(s.c_str()); } // <-- swapped order
void LogMsg(const char *msg){ // <-- swapped order
Serial.println(msg);
}
void setup() {
// put your setup code here, to run once:
Serial.begin(115200);
Serial.println("reset");
}
void loop() {
// put your main code here, to run repeatedly:
String str3 = "testing string cat ";
uint32_t var = 12345;
LogMsg(str3 + var);
delay(500);
}
Edit: The definition of void LogMsg(String s) was changed to reflect error in original and the simplification suggested by #hcheung. Behavior remains the same. The first instance works, the second crashes.
C strings are terminated with '\0'. So toCharArray() will append a null character to your Ardunio String. Otherwise you would have to provide a length with the char pointer everytime you want to use that string.
Your char array must be big enough to fit this extra character or you will cause an access violation if toCharArray does not throw an exception first.
I have written the following code:
void dice (int &x) {
for (int i = 0;i<7;i++){
delay (35);
int kocka;
kocka = random (1,7);
randomSeed (analogRead (A7));
delay(5);
}
}
void setup() {
Serial.begin(9600);
}
void loop() {
Serial.println(dice(int x));
};
However when I try to compile it, I get this error:
expected primary-expression before 'int'
You can call a void function, but for the argument you pass you don't put "int". You know this if you think about it, when you write a digitalWrite do you put "int" in front of the pin number? You only need the int when you are writing the function prototype.
You also don't have any variable x defined to pass to that function, so your next error is "x not declared in this scope" since in the loop function there is no x variable.
You have another problem lurking next. Your function is defined to return void. That means it returns nothing. And sure enough, there is no return statement. So when you remove the "int" in the Serial.print line, and define a variable x, you will get a "void value not ignored" error. You can't print the return value from a function that has no return value.
It isn't really clear what you want that to print out. Edit your question to say what you want to happen and maybe someone can help you figure out how to do it.
In the following piece of code, I'm retrieving a shared_ptr<A> from a function. I then dynamically cast the pointer to a deriving class and store it in a shared_ptr<B>. The original pointer is not a nullptr.
shared_ptr<B> storage = nullptr;
if (...)
{
shared_ptr<A> definition = getSharedPointer();
// Store the lambda
storage = dynamic_pointer_cast<B>(definition);
}
I would expect the dynamic_pointer_cast and storage to storage to increase the total reference count to 2. Then, when I leave the scope of the if-statement, storage's reference count should be one.
Yet, when I tried to call a method on storage, I get a EXC_BAD_ACCESS, implying I'm reading in a deleted pointer.
storage->foo(...)->bar(...);
Is my logic wrong? Is this a bug in clang (can't imagine)?
EDIT
I seem to have found the error, which has nothing to do with the pointers. The function bar() actually gave the problem. If anyone ever reads this: the above code is perfectly valid.
This example works fine:
#include <memory>
using namespace std;
struct A {
virtual ~A() {}
};
struct B : A {};
shared_ptr<A> getSharedPointer() {
return make_shared<B>();
}
#include <iostream>
int main() {
shared_ptr<B> storage = nullptr;
if (true)
{
shared_ptr<A> definition = getSharedPointer();
// Store the lambda
storage = dynamic_pointer_cast<B>(definition);
}
cout << storage.get() << endl;
}
It would seem that your shared_ptr<A> is not pointing to a B and the result of the dynamic_pointer_cast is nullptr. Maybe a debugging statement would be helpful:
if (...)
{
shared_ptr<A> definition = getSharedPointer();
cerr << "as A: " << definition.get()
<< ", as B: " << dynamic_cast<B>(definition.get()) << endl;
// Store the lambda
storage = dynamic_pointer_cast<B>(definition);
}
I am sorry that I cannot support my question with some code (I didnt understand how to structure it so it would be accepted here), but I try anyway.
If I understand correctly, a struct that references a struct of same type would need to do this with contained pointer for reference. Can this pointer reference to allocated space on the stack (instead of the heap) without creating segmentation fault? -
how should this be declared?
Yes, you can use pointers to variables on the stack, but only when the method that provides that stack frame has not returned. For example this will work:
typedef struct
{
int a;
float b;
} s;
void printStruct(const s *s)
{
printf("a=%d, b=%f\n", s->a, s->b);
}
void test()
{
s s;
s.a = 12;
s.b = 34.5f;
printStruct(&s);
}
This will cause an error however, as the stack frame would have disappeared:
s *bad()
{
s s;
s.a = 12;
s.b = 34.5f;
return &s;
}
EDIT: Well I say it will cause an error, but while calling that code with:
int main()
{
test();
s *s = bad();
printStruct(s);
return 0;
}
I get a warning during compilation:
s.c:27:5: warning: function returns address of local variable [enabled by default]
and the program appears to work fine:
$ ./s
a=12, b=34.500000
a=12, b=34.500000
But it is, in fact, broken.
You didn't say what language you are working in, so assuming C for now from the wording of your question... the following code is perfectly valid:
typedef struct str_t_tag {
int foo;
int bar;
struct str_t_tag *pNext;
} str_t;
str_t str1;
str_t str2;
str1.pNext = &str2;
In this example both str1 and str2 are on the stack, but this would also work if either or both were on the heap. The only thing you need to be careful of is that stack variables will be zapped when they go out of scope, so if you had dynamically allocated str1 and passed it back out of a function, you would not want str1->pNext to point to something that was on the stack within that function.
In other words, DON'T DO THIS:
typedef struct str_t_tag {
int foo;
int bar;
struct str_t_tag *pNext;
} str_t;
str_t *func(void)
{
str_t *pStr1 = malloc(sizeof(*pStr1));
str_t str2;
pStr1->pNext = &str2;
return pStr1; /* NO!! pStr1->pNext will point to invalid memory after this */
}
Not sure if this is specifically a C/C++ question, but I'll give C/C++ code as example in anyway.
The only way you can declare it: (with minor variations)
typedef struct abc
{
struct abc *other;
} abc;
other can point to an object on the stack as follows:
abc a, b; // stack objects
b.other = &a;
This is not a question about scope, so I'll skip commenting on possible issues with doing the above.
If, however, you want to assign it to a dynamically created object, there's no way this object can be on the stack.
abc b;
b.other = malloc(sizeof(abc)); // on the heap