I'm getting an unexpected behavior when passing a const string to a function as a parameter.
I have this function:
std::string getStatusByTopic(const std::string topic){
if (topic.compare(TOPIC_WP_SM_CMD) == 0)
return state_to_string(STATUSES::WP_HANDLER_STATUS);
}
Which is not totally implemented yet.
And I have this function:
InsertStatus(getStatusByTopic(TOPIC_WP_SM_CMD),msg);
Which declared as:
void Msp::InsertStatus(const std::string status_topic, const std_msgs::String::ConstPtr& new_status)
I have 2 questions:
These functions have been compiled successfully, even though there's a possible usage where getStatusByTopic() returns nothing when its defined returned type is string. How is it possible?
When the code is executed, with the scenario that and getStatusByTopic(TOPIC_WP_SM_CMD) returns nothing. There's no runtime errors, more over - TOPIC_WP_SM_CMD is passed as parameter to InsertStatus() (which is defined ad "/some/topic/"). How is that possible?
Thanks.
Related
I'm working on a project in C++, but at some point in the application it fails and generates a core dump. The application uses a couple of classes, which for the purposes here I'm concentrating on one of the classes, which I'm calling A, and is instantiated as object a. This has a large number of member functions, of which at the moment only a few are being used, but one of these generates a log to produce diagnostics to be used for debugging. I want to use this to find out why the application is failing.
The project is to put together code that invokes the various member functions, and although I have access to the source code and some limited documentation, none of the code can be changed, with all changes being in the code that makes use of the classes and invokes the member functions. The member function in question is:
void enable_log (log_callback callback, void * user_data = nullptr)
where the 1st argument callback contains the message and 2nd argument is optional. For now it can be set to nullptr, so would be invoked as:
a.enable_log(callback, nullptr);
From this documentation it's not at all clear what exactly callback is. However, in looking at the source code this is:
using log_callback = void (*)(const std::string& message, void* user_data);
in a header file, where log_callback is an alias for const std::string& if I understand this correctly.
I already have dummy classes on a platform using Visual Studio 2019 with some test member functions to simulate invoking the member functions on a remote Linux server, but I'm unable to find a way of making use of the member function above. I added the test member function to the dummy class as follows:
void enable_log(const std::string& callback, void* user_data = nullptr) {
callback = "ABCD";
}
which is supposed to generate a test string which is returned, such that in the real application this string will have diagnostic information that will be written to a file. However, the "=" is an error.
The idea is that in the main function an empty string will be declared, then enable_log() should populate this string, which can be printed out.
I've spent some time looking at various resources, including Stackoverflow, but I cannot find a way of returning a string with the information that can be printed out. I need a simple way to simulate this, and as I said above, I must not change the source code of the real member function, so the simulated member function has to produce a string in the same way. How is this done? Some advice would be appreciated.
Callback, in simple words, is some function that will be called later at some point. Example:
void callback_fn(int a);
using callback_t = (void)(*)(int a);
void some_func(callback_t);
You can use some_func() like so:
some_func(callback_fn);
Full example here: https://godbolt.org/z/ET3GhfYrv
For your usecase the parameters of the callback are slightly different. Here's how to read the syntax:
using log_callback = // this just creates an alias for whatever is on the right handside
void // the return type of the "callable" should be void
(*) // this tells us that it is a function pointer
(const std::string& message, void* user_data) // These are the arguments the callable takes. It is a "std::string" and a "void *"
To use this, just create a free function with the same signature:
void callable(const std::string &msg, void *userData = nullptr)
{
// msg is the data sent by the function. use it in whatever way
// you want.
std::cout << msg << '\n';
}
// Pass it to the enable_log
enable_log(callable);
I have an assignment where i was given c++ code that has multiple test functions, and i have to write the stri class with all the functions necesary for these tests to pass. I am having difficulty with one particular test about chain linking.
void Lab4Tests::testChainedSet() {
stri greet = "Hello";
assert(0==strcmp("Hello", greet.get()), "testChainedSet", "internal representation error");
greet.set("Hi").set("Buna");
assert(0==strcmp("Buna", greet.get()), "testChainedSet", "set failed");
}
My function inside the class that I wrote to solve this test is the following
char* set(const char* s){
len=strlen(s);
repres=new char(len+1);
strcpy(repres,s);
return repres;
}
I get an error when compiling the code
error: request for member 'set' in 'greet.stri::set(((const char*)"Hi"))', which is of non-class type 'char*'|
I dont understand chain linking that well, I would appreciate if somebody could point out what I'm doing wrong. Thanks in advance!
If you want to be able to chain calls you need to return a reference to the this object.
For instance
stri& set(const char* s)
{
repres = std::strdup(s);
return *this;
}
I've taken the liberty of simplifying your string duplication. I haven't fixed the memory leak that you will get if you call set when repres already contains dynamically allocated memory.
I have some code that looks like this:
static std::string Foo(const std::string& blah)
{
if ( someWierdEdgeCase() ){
return false; // <-- this line has a compiler error
}
std::string resourcePath;
resourcePath.append("/assets/");
return resourcePath;
}
It used to compile fine, but then I upgraded to Xcode 5.1, and now it no longer compiles. The error I get is the following one:
No viable conversion from 'bool' to 'std::string'
I would like to know what I should replace 'return false' with so that this works the way it used to.
Normally, I would be happy to return something other than false, and clearly that is not good behavior. But somehow this code used to compile and run, and I want to understand what it was doing, in case some other part of this codebase is relying on what is certainly weird behavior.
This:
return false; // <-- this line has a compiler error
There is no standard way to convert bool to std::string (please correct me if there is or was something special in gcc (the old XCode mac compiler)). This means that your code base used to contain explicit code to convert the bool to string.
If it is not compiling now this suggests this conversion was removed from your code base.
A couple of people of suggested alternatives. But I doubt any of these are going to work. As the old code had a depedency on how it used to work. So making any specific recomendation will depend on working out how the old code compiled and what it returned when someWierdEdgeCase() is true. Basically you need to replicate this behavior.
Otherwise you need to hunt down all used cases in your code and change the behavior to match the new behavior. In this case I would change the name of the function. Re-Compile and see where the code breaks and check the behavior at each location and make sure it behaves the same.
static std::string Foo(const std::string& blah)
{
std::string resourcePath = "";
if ( someWierdEdgeCase() ){
return resourcePath; // <-- Now you're returning a string
}
resourcePath.append("/assets/");
return resourcePath;
}
If you need the return type to be - for whatever reason - not always present, return by pointer, instead of returning by value.
static yourType* Foo(const std::string& blah){
if ( someWierdEdgeCase() ){
return 0;
}
}
Then you can test and assign the function in the same line
yourType* result;
if(result = Foo("something")){
// the result was correct
Of course - since your function returns a std::string, you can return an empty string (""), or - also independent of the return type - throw an exception.
I had same issue that with the fact that my interface file and implementation file were using different data types in method declaration.Putting same data type on both place in method declaration error went away.
I would like to call the following code in C++, which I cannot change:
void getAge(char *name)
{
// do something
}
When I call it with getAge("hello");, it has the following warning:
warning: deprecated conversion from string constant to 'char*'
but there is no warning in C code. What is the difference, and how do I change the call to avoid the warning in C++?
the function […] can not be changed
Then write a wrapper around the function and copy the string – or, if you feel lucky (= you know that the string won’t be modified inside the original function), explicitly cast away const-ness:
void getAge(char const* name) {
the_namespace::getAge(const_cast<char*>(name));
}
If you’re unsure whether the function modifies its parameters, use something like the following – however, if that’s the case then calling the function with a string literal (getAge("hello")) would have been invalid anyway.
void getAge(char const* name) {
std::string buffer(name);
the_namespace::getAge(&buffer[0]);
}
Here we copy the string into a modifiable buffer and pass an address to its first character to the original function.
The safest way is to copy the string, then call the C function:
void getAgeSafe(const char* name)
{
std::vector<char> tmp = name?
std::vector<char>(name, name+1+strlen(name))
:std::vector<char>();
getAge( tmp.data() );
}
and call getAgeSafe from your C++ code.
A less safe way that relies on the C code never modifying the char* name would be to const_cast, again in a "wrapping" function:
void getAgeUnsafe(const char* name)
{
getAge( const_cast<char*>(name) );
}
but this time the name is more scary, as is the operation. If you call getAge with a compile time constant string like "bob", if getAge modifies its input, undefined behavior results (this is true in both C and C++ -- C++ at least warns you about it).
You can try getAge((char*)"hello").
In c++ you can write it like this,
void getAge(string name)
{
// do something
}
and also include the header file #include<string> because you are using string now
I am learning C++ and very new at using classes, and I am getting very confused in trying to use them. I am trying to convert my existing code (which used structs) so that it uses classes - so while I know what I am trying to do I don't know if I'm doing it correctly.
I was told that when using functions from the class, I first need to instantiate an object of the class. So what I have tried (a snippet) in my main function is:
int main()// line 1
{
string message_fr_client = "test"; //line2
msgInfo message_processed; //line 3
message_processed.incMsgClass(message_fr_client); //line 4
if (!message_processed.priority_check(qos_levels, message_processed)) //line 5
cout << "failure: priority level out of bounds\n"; //line 6
return 0; //line 7
}
Could you help me clarify if my following assumptions are correct? The compiler is not showing any error and so I don't know if it is error-free, or if there are ones lurking beneath.
At line 4, is the function incMsgClass being performed on the string message_fr_client and returning the resultant (and modified) message_processed?
At line 5, the function priority_check is being performed on the message_processed and returning a boolean?
In my class definition, I have a function getPath that is meant to modify the value of nodePath - is it just a matter of using message_processed.getPath(/*arguments*/)?
I haven't included the body of the functions because I know they work - I would just like to find out how the class functions interact. Please let me know if I can be clearer - just trying to clear up some confusion here.
Here is my class:
#ifndef clientMsgHandling_H
#define clientMsgHandling_H
#include <list>
#include <map>
#include <queue>
class msgInfo
{
public:
msgInfo();
msgInfo(int, int, int, std::string, std::list<int>);
/*classifying message*/
msgInfo incMsgClass(std::string original_msg);
/*message error checks*/
bool priority_check(int syst_priority, msgInfo msg); //check that message is within qos levels
bool route_check(std::map<std::pair<int, int>, int> route_table, msgInfo msg); //check that route exists
void getPath(msgInfo msg, std::map<std::pair<int, int>, int> route_info, int max_hop);
private:
int source_id;
int dest_id;
int priority;
std::string payload;
std::list<int> nodePath;
};
#endif
While it may compile (and even run), there are a few oddities with the code as shown:-
First off, class methods know which object they are operating on - so your priority_check and route_check methods probably don't need msgInfo as a parameter.,
for example, your old non-class function might be like this
bool priority_check(int p, msgInfo msg)
{
return msg.priority < p;
}
But the new one should look like this:
bool msgInfo::priority_check(int p)
{
return priority < p;
}
Also, incMsgClass is a bit odd, as it's a non-static class method that returns a msgInfo object. It's difficult to tell without understanding what it's supposed to do, but it seems possible that this function should actually be a constructor, rather than a regular method.
One other thing is that you're currently passing a msgInfo by value to those methods. So if the method needed to modify the passed msgInfo, it would not have any effect. It's generally better to pass objects by reference or const reference to other methods. So, back to the previous non-method example, it should really be this.
bool priority_check(int p, const msgInfo &msg)
...
But, as I said, you probably don't need the msgInfo parameters anyway.
At line 4, is the function incMsgClass being performed on the string message_fr_client
Yes
and returning the resultant (and modified) message_processed?
Whatever it's returning, you're ignoring the return value. It can modify the object itself, yes, because the function is not const.
At line 5, the function priority_check is being performed on the message_processed and returning a boolean?
Yes
In my class definition, I have a function getPath that is meant to modify the value of nodePath - is it just a matter of using message_processed.getPath(/arguments/)?
If a member function is intended to modify one of the class members, it's just a matter of not marking that function const
Hard to tell without implementation-details, but here we go:
I. You are passing a std::string as value (C++ is call-by-value by default), so you get a copy of the std::string in your method. If you want to work on the object you passed and manipulate it, use a reference on the object, like
msgInfo incMsgClass(std::string& original_msg); // notice the ampersand
then you can change your signature to
void incMsgClass(std::string& original_msg);
as you don't need to return the std::string you passed.
II. Yes, at least according to your signature
III. Can see a node_path only as a member.
For all your questions, see C++-FAQ.
Your basic assumptions are correct.
message_processed.incMsgClass(message_fr_client); //line 4
This line is not correct. The function you call returns msgInfo which is simply dropped. You should assign it to something. But it is not as it is usually done. You should make it a constructor of msgInfo, like
class msgInfo
{
public:
msgInfo(std::string original_msg);
...
}
Then you could call it like this
msgInfo message_processed(message_fr_client);
That line would create a msgInfo that is already properly initialized.
There is another pattern for creating class instances - static creating function. In your case you could mark incMsgClass static and then call it like
msgInfo message_processed = msgInfo.incMsgClass(message_fr_client);
I seriously doubt you need this pattern here, so I'd advise to move to constructor.
As of other functions, I see no problems there. Just note that all member functions not marked as const can modify the object they are called on. So, you don't need to pass this object explicitly. For functions a pointer to the object they are called on is available by name this. Also the functions can access all class variables as if these variables are global for normal (non-member) functions.