Why is ParseFromString() not producing an output in my application using protobuf? - c++

I currently have a *.cpp file where I import the needed *.proto.h file and instantiate an object based on one of the messages(Heartbeat) from my *.proto file. My goal is to use the set_value(), SerializeToString(), and ParseFromString().I've had success accomplishing the first two tasks. When using ParseFromString() not output is produced to the screen. I'm pretty new to protobufs so there may be something I'm overlooking. This is my code.
#include <iostream>
#include <fstream>
#include <string>
#include "cpnt.pb.h"
using namespace std;
int main()
{
wombat::HeartBeat h;
string m;
string t;
string v;
h.set_value(1);
v = h.SerializeToString(&m);
t = h.ParseFromString(v);
cout << "\n The message received is:" << h.value();
cout << "\n The binary data stored in v is:" << v;
cout << "\n The parsed data stored in t is:" << t <<::endl;
return 0;
}
And this is a printout of the output:

You misunderstood how SerializeToString works. h.SerializeToString(&m); serializes h and stores the result in m. h.ParseFromString(v); parsed from the string v to set members of h. Hence it should be
h.set_value(1);
h.SerializeToString(&m);
h.ParseFromString(m);
Both functions retun a bool, but I admit that I didn't find what it is good for in the documentation. Probably to signal an error.

Related

VS code set up for c++

I am new in VS code. I wrote a C++ code like one below. but unfortunately in the terminal or output panel I cannot get both of the string and variable value. in the terminal only variable's inputted value is showing. How to fix this?
#include <bits/stdc++.h>
int main()
{
int slices;
std::cin >> slices;
std::cout << "You got " << slices << " of pizzas" << std::endl;
return 0;
}

Why does the function find of C++ stl string sometimes go wrong sometime go right?

I am trying to do some file reading with C++ in Ubuntu 16.04 (GCC&G++ 5.4 and CMake 3.5.1).
The test file (named 123.txt) have only a line words just like this:
Reprojection error: avg = 0.110258 max = 0.491361
I just want to get the avg error and max error. My method is to get a line and put them into a std::string
and use string::find. My codes are very easy just like this:
#include <iostream>
#include <string>
#include <stdio.h>
using namespace std;
int main()
{
FILE *fp = fopen("123.txt", "r");
char tmp[60];
string str;
fgets(tmp, size_t(tmp), fp);
fclose(fp);
cout << tmp << endl;
str = tmp;
cout << str.size() << endl;
size_t avg = str.find("avg");
size_t max = str.find("max");
cout << avg << endl;
cout << max << endl;
}
I can use g++ to compile it successfully. But I meet a strange issue.
When I first run it in the command, it will get the right result:
Reprojection error: avg = 0.110258 max = 0.491361
52
20
37
If I run codes again, it will go wrong sometimes just like this:
p
2
18446744073709551615
18446744073709551615
The "p" is a disorderly code which can not be shown correctly in the command. I am not good at C++ and feel confused about it. Is there someone who can say something? Thank you!
The expression
fgets(tmp, size_t(tmp), fp);
is ill-formed, size_t(tmp) will not work as you expect, you need sizeof(tmp).
The 52 value you get is because fgets consumes the \n character and this is counted too, actually the string has 51 characters counting with spaces.
That said, in this case you can use better C++ tools to replace the C ones you are using, fopen can be replaced by using the fstream library, fgets can be replaced by getline.
Something like:
#include <iostream>
#include <string>
#include <fstream>
int main()
{
std::ifstream fp("123.txt"); //C++ filestream
if (fp.is_open()) {//check for file opening errors
std::string str;
std::getline(fp, str); //C++ read from file
fp.close();
std::cout << str << std::endl;
std::cout << str.size() << std::endl;
size_t avg = str.find("avg");
size_t max = str.find("max");
std::cout << avg << std::endl;
std::cout << max << std::endl;
}
else{
std::cerr << "Couldn't open file";
}
}
Note that I dind't use using namespace std;, this is for a reason, it's not a good practice, you can check this thread for more details.

Does it make sense to apply make_move_iterator together with vector::insert ?(C++)

I have a vector of vectors, and I want to connect them one by one to form a long vector. This could be done by inserting at the end. Inspired by this question, I was thinking that using make_move_iterator would replace copy with move and thus would be more efficient. But the following test demonstrates that make_move_iterator will cause a larger time consumption.
#include <iostream>
#include <string>
#include <vector>
#include <chrono>
using namespace std;
int main()
{
string a = "veryveryveryveryveryveryveryveryveryveryveryveryveryveryveryveryveryveryveryveryveryveryveryveryveryveryveryverylongstring";
vector<string> b(10,a);
vector<vector<string> > c(1000,b);
vector<string> d,e;
auto t1 = chrono::system_clock::now();
for(auto& item : c)
{
d.insert(d.end(),item.begin(),item.end());
}
cout << c[0][0].length() << endl;
auto t2 = chrono::system_clock::now();
for(auto& item:c)
{
e.insert(e.end(), std::make_move_iterator(item.begin()),std::make_move_iterator(item.end()));
}
auto t3 = chrono::system_clock::now();
cout << chrono::duration_cast<chrono::nanoseconds>(t2-t1).count() << endl;
cout << chrono::duration_cast<chrono::nanoseconds>(t3-t2).count() << endl;
cout << c[0][0].length() << endl;
cout << "To check that c has been moved from." <<endl;
}
//Output:
//122
//1212000
//1630000
//0
//To check that c has been moved from.
Thus I'm wondering, does this approach really help improve efficiency?
The test in the question description was conducted on cpp shell
I later tried on ideone and it turned out that make_move_iterator is obviously more efficient. So it seems to be a compiler-dependent thing.
122
320576
98434
0
To check that c has been moved from.

Run std::function getted by binary read

I'm developing a application and my idea is store "apps" in files, like executables. Now i have that:
AppWriter.c
#include <vector>
#include <time.h>
#include <functional>
struct PROGRAM
{
std::vector<int> RandomStuff;
std::vector<std::function<void()>> Functions;
std::function<void()> MAIN;
} CODED;
void RANDOMFUNC()
{
srand(time(NULL));
for(int i = 0; i < 40; i++)
CODED.RandomStuff.push_back(rand() % 254);
}
void LOGARRAY()
{
for(int i = 0; i < CODED.RandomStuff.size(); i++)
std::cout << "["<< i + 1 <<"]: "<< CODED.RandomStuff[i] << std::endl;
}
void PROGRAMMAIN()
{
std::cout << "Hello i call random function!" << std::endl;
CODED.Functions[0]();
CODED.Functions[1]();
}
void main()
{
CODED.MAIN = PROGRAMMAIN;
CODED.Functions.push_back(RANDOMFUNC);
CODED.Functions.push_back(LOGARRAY);
std::cout << "Testing MAIN" << std::endl;
CODED.MAIN();
FILE *file = fopen("TEST_PROGRAM.TRI","wb+");
fwrite(&CODED,sizeof(CODED),1,file);
fclose(file);
std::cout << "Program writted correctly!" << std::endl;
_sleep(10000);
}
AppReader.c
#include <iostream>
#include <vector>
#include <time.h>
#include <functional>
struct PROGRAM
{
std::vector<int> RandomStuff;
std::vector<std::function<void()>> Functions;
std::function<void()> MAIN;
} DUMPED;
void main()
{
FILE *file = fopen("TEST_PROGRAM.TRI","rb+");
fseek(file,0,SEEK_END);
int program_len = ftell(file);
rewind(file);
fread(&DUMPED,sizeof(PROGRAM),1,file);
std::cout
<< "Function array size: " << DUMPED.Functions.size() << std::endl
<< "Random Stuff Array size: " << DUMPED.RandomStuff.size() << std::endl;
DUMPED.MAIN();
}
When i run AppReader the functions dont work(Maybe why std::function it's like void pointers?), but in arrays or if i add variables i can see with debugger the data are storaged correctly (for that i tryed the vector of functions), but whatever doesn't work throw's me error on functional file. ¿Any ideas how i can do that?
This is never going to work. At all. Ever. std::function is a complex type. Binary reads and writes don't work for complex types. They never can. You would have to ask for functions in a pre-defined serializable format, like LLVM IR.
Your problem is that you're storing information about functions that exist in one executable, then trying to run them in a separate executable. Other than that, your code does work, but as DeadMG says, you shouldn't be storing complex types in a file. Here's how I modified your code to prove that your code works if run within a single executable:
#include <iostream>
#include <vector>
#include <time.h>
#include <functional>
struct PROGRAM
{
std::vector<int> RandomStuff;
std::vector<std::function<void()>> Functions;
std::function<void()> MAIN;
} CODED;
void RANDOMFUNC()
{
srand(time(NULL));
for(int i = 0; i < 40; i++)
CODED.RandomStuff.push_back(rand() % 254);
}
void LOGARRAY()
{
for(int i = 0; i < CODED.RandomStuff.size(); i++)
std::cout << "["<< i + 1 <<"]: "<< CODED.RandomStuff[i] << std::endl;
}
void PROGRAMMAIN()
{
std::cout << "Hello i call random function!" << std::endl;
CODED.Functions[0]();
CODED.Functions[1]();
}
int main()
{
CODED.MAIN = PROGRAMMAIN;
CODED.Functions.push_back(RANDOMFUNC);
CODED.Functions.push_back(LOGARRAY);
std::cout << "Testing MAIN" << std::endl;
CODED.MAIN();
FILE *file = fopen("TEST_PROGRAM.TRI","wb+");
fwrite(&CODED,sizeof(CODED),1,file);
fclose(file);
std::cout << "Program writted correctly!" << std::endl;
// _sleep(10000);
std::cout << "---------------------\n";
file = fopen("TEST_PROGRAM.TRI","rb+");
fseek(file,0,SEEK_END);
int program_len = ftell(file);
rewind(file);
fread(&CODED,sizeof(PROGRAM),1,file);
std::cout
<< "Function array size: " << CODED.Functions.size() << std::endl
<< "Random Stuff Array size: " << CODED.RandomStuff.size() << std::endl;
CODED.MAIN();
}
The problem is not that you're storing complex types via binary read/write, per se. (Although that is a problem, it's not the cause of the problem you posted this question about.) Your problem is that your data structures are storing information about the functions that exist in your 'writer' executable. Those same functions don't even exist in your 'reader' executable, but even if they did, they likely wouldn't be at the same address. Your data structures are storing, via std::function, pointers to the addresses where the functions exist in your 'writer' executable. When you try to call these non-existent functions in your 'reader' executable, your code happily tries to call them but you get a segfault (or whatever error your OS gives) because that's not the start of a valid function in your 'reader' executable.
Now with regard to writing complex types (e.g. std::vector) directly to a file in binary format: Doing so "works" in the sample code above only because the binary copies of the std::vectors have pointers that, once read back in, still point to valid data from the original std::vectors which you wrote out. Note that you didn't write the std::vector's actual data, you only wrote their metadata, which probably includes things like the length of the vector, the amount of memory currently allocated for the vector, and a pointer to the vector's data. When you read that back, the metadata is correct except for one thing: Any pointers in it are pointing to addresses that were valid when you wrote the data, but which may not be valid now. In the case of the sample code above, the pointers end up pointing to the same (still valid) data from the original vectors. But there's still a problem here: You now have more than one std::vector that thinks they own that memory. When one of them is deleted, it will delete the memory that the other vector expects to still exist. And when the other vector is deleted, it will cause a double-delete. That opens the door to all kinds of UB. E.g. that memory could have been allocated for another purpose by that time, and now the 2nd delete will delete that other purpose's memory, or else the memory has NOT been allocated for another purpose and the 2nd delete may corrupt the heap. To fix this, you'd have to serialize out the essence of each vector, rather than their binary representation, and when reading it back in, you'd have to reconstruct an equivalent copy, rather than simply reconstitute a copy from the binary image of the original.

"Roll-Back" or Undo Any Manipulators Applied To A Stream Without Knowing What The Manipulators Were [duplicate]

This question already has answers here:
Restore the state of std::cout after manipulating it
(9 answers)
Closed 4 years ago.
If I apply an arbitrary number of manipulators to a stream, is there a way to undo the application of those manipulators in a generic way?
For example, consider the following:
#include <iostream>
#include <iomanip>
using namespace std;
int main()
{
cout << "Hello" << hex << 42 << "\n";
// now i want to "roll-back" cout to whatever state it was in
// before the code above, *without* having to know
// what modifiers I added to it
// ... MAGIC HAPPENS! ...
cout << "This should not be in hex: " << 42 << "\n";
}
Suppose I want to add code at MAGIC HAPPENS that will revert the state of the stream manipulators to whatever it was before I did cout << hex. But I don't know what manipulators I added. How can I accomplish this?
In other words, I'd like to be able to write something like this (psudocode/fantasy code):
std::something old_state = cout.current_manip_state();
cout << hex;
cout.restore_manip_state(old_state);
Is this possible?
EDIT:
In case you're curious, I'm interested in doing this in a custom operator<<() I'm writing for a complex type. The type is a kind of discriminated union, and different value types will have different manips applied to the stream.
EDIT2:
Restriction: I cannot use Boost or any other 3rd party libraries. Solution must be in standard C++.
Yes.
You can save the state and restore it:
#include <iostream>
#include <iomanip>
using namespace std;
int main()
{
std::ios state(NULL);
state.copyfmt(std::cout);
cout << "Hello" << hex << 42 << "\n";
// now i want to "roll-back" cout to whatever state it was in
// before the code above, *without* having to know what modifiers I added to it
// ... MAGIC HAPPENS! ...
std::cout.copyfmt(state);
cout << "This should not be in hex: " << 42 << "\n";
}
If you want to get back to the default state you don't even need to save the state you can extract it from a temporary object.
std::cout.copyfmt(std::ios(NULL));
The standard manipulators all manipulate a stream's format flags, precision and width settings. The width setting is reset by most formatted output operations anyway. These can all be retrieved like this:
std::ios_base::fmtflags saveflags = std::cout.flags();
std::streamsize prec = std::cout.precision();
std::streamsize width = std::cout.width();
and restored:
std::cout.flags( saveflags );
std::cout.precision( prec );
std::cout.width( width );
Turning this into an RAII class is an exercise for the reader...
Saving and restoring state is not exception-safe. I would propose to shuffle everything into a stringstream, and finally you put that on the real stream (which has never changed its flags at all).
#include <iostream>
#include <iomanip>
#include <sstream>
int main()
{
std::ostringstream out;
out << "Hello" << std::hex << 42 << "\n";
std::cout << out.str();
// no magic necessary!
std::cout << "This should not be in hex: " << 42 << "\n";
}
Of course this is a little less performant. The perfect solutions depends on your specific needs.
Boost IO State saver might be of help.
http://www.boost.org/doc/libs/1_40_0/libs/io/doc/ios_state.html
I know that is an old question, but for future generations:
You can also write a simple state saver yourself (it will certainly help you avoid leaving the state changed). Just use the solution suggested by #loki and run it from the constructor/destructor of an object (in short: RAII) along these lines:
class stateSaver
{
public:
stateSaver(ostream& os): stream_(os), state_(nullptr) { state_.copyfmt(os); }
~stateSaver() { stream_.copyfmt(state_); }
private:
std::ios state_;
ostream& stream_;
};
Then, you will use it like this:
void myFunc() {
stateSaver state(cout);
cout << hex << 42 << endl; // will be in hex
}
int main() {
cout << 42 << endl; // will be in dec
myFunc();
cout << 42 << endl; // will also be in dec
}