Optionally print to stdout at runtime - c++

I have output in a C++ program that I only want to see if a "verbose" option is specified at runtime. I've found lots of methods to use preprocessor #define statements to control debugging output when compiling, but I can't find any ways to do this at runtime short of wrapping every cout in if(verbose).
In pseudocode, I'd like to transform:
if(verbose)
cout << "Some text: " << variable << endl;
...
if(verbose)
cout << "Other text: " << var << endl;
Into:
if(verbose)
//block cout
cout << "Some text: " << variable << endl;
cout << "Other text: " << var << endl;
Is there some way to optionally redefine cout at runtime so those lines silently print nothing? Better yet would be a more flexible approach that allows some output while blocking others.

You can simply bind a stream object reference to different streams depending on verbose:
ostream& vout = (verbose? cout : nullstream);
Then use vout for all the output that should only appear when verbose.
Of course you need to define nullstream first; that involves defining a streambuf derived class.

I'm not sure whether you're in the process of writing a program, or whether you already have a program you want to disable output for. Generally, when you're writing software with debugging output, you'll want to create some sort of logging class to help you control when output occurs and where it goes (e.g., to a file, to stdout, to stderr, etc.). An example of a simple one would be:
#include <iostream>
class Logger {
public:
Logger(bool enabled) : enabled(enabled) { }
void setEnabled(bool enabled) { this->enabled = enabled; }
template<typename T> Logger& operator<<(T const& t)
{
if (enabled)
std::cout << t;
return *this;
}
// If you'd like manipulator support (e.g., for std::endl)
Logger& operator<<(std::ostream& (*manipulator)(std::ostream&))
{
if (enabled)
std::cout << manipulator;
return *this;
}
private:
bool enabled;
};
int main()
{
Logger log(true);
log << "Hello, " << "World!" << 123; // Prints Hello, World!123
log.setEnabled(false);
log << "Hello, " << "World!" << 123; // Prints nothing
}
Alternatively, you could redirect cout, as others have mentioned, or even just simply redirect your program's output to /dev/null on the command line:
./myProgram > /dev/null

Related

Deferring cout output until just prior to the next output

I have some C++ console programs that display progress information on the last line of output, at regular intervals.
This progress line is cleared prior to writing the next real output (or updated progress information); this could be from a number of different places in the source, and I'm currently clearing the progress line on each one, e.g.:
cout << clearline << "Some real output" << endl;
...
cout << clearline << "Some other real output" << endl;
...
cout << clearline << setw(4) << ++icount << ") " << ... << endl;
...
cout << clearline << "Progress info number " << ++iprog << flush;
Here, 'clearline' is some (system dependent) string like "\r\33[2K" which clears the current last line.
I would prefer something cleaner, that localises source changes to the actual line that's going to be cleared, like simply:
cout << "Progress info number " << ++iprog << flush << defer_clearline;
where 'defer_clearline' causes the writing of 'clearline' to be deferred until just prior to the next cout output, wherever and whatever that happens to be. I then wouldn't need to use 'clearline' on all the other lines.
I thought it might be possible to do this if 'defer_clearline' is a manipulator, and/or using xalloc() and iword().
But I've not managed to get anything that works.
Is it possible to do this sort of thing, and if so how?
2020-12-30: edited to include missing 'flush's.
You can pretty easily setup an std::cout wrapper:
// Declare the empty struct clear_line and instantiate the object cls
struct clear_line { } cls;
class out {
private:
std::ostream &strm = std::cout;
bool is_next_clear = false;
public:
template <typename T>
out& operator<<(const T& obj) {
if(is_next_clear) {
strm << std::endl << std::endl << std::endl; // clear logic
is_next_clear = false;
}
strm << obj;
return *this;
}
out& operator<<(const clear_line& _) {
is_next_clear = true;
return *this;
}
};
This pretty simply stores an is_next_clear bool for whether or not the next regular output should be cleared. Then, in the general case (the templated operator<<()), we run your clear logic and flip the is_next_clear flag if applicable. Then just output as usual.
Then, the operator<<() is overloaded for the case of a clear_line object. So if one of those is sent, we know to flip the is_next_clear flag, but not actually output anything.
Here's an example use:
int main() {
out o;
o << "Some real output" << cls;
o << "Some other real output";
return 0;
}
Here it is in action: https://ideone.com/0Dzwlv
If you want to use endl, you'll need to add a special overload for it as this answer suggests: https://stackoverflow.com/a/1134467/2602718
// this is the type of std::cout
typedef std::basic_ostream<char, std::char_traits<char> > CoutType;
// this is the function signature of std::endl
typedef CoutType& (*StandardEndLine)(CoutType&);
// define an operator<< to take in std::endl
out& operator<<(StandardEndLine manip)
{
// call the function, but we cannot return its value
manip(strm);
return *this;
}
Live example: https://ideone.com/ACUMOo

{fmt} equivalent to cout.rdbuf?

I have a unit test that verifies a function by reading the buffer sent by the function:
template <typename Manifold>
void print_manifold(Manifold const& manifold)
try
{
std::cout << "Manifold has " << manifold.N0() << " vertices and "
<< manifold.N1() << " edges and " << manifold.N2() << " faces and "
<< manifold.N3() << " simplices.\n";
// fmt::print(
// "Manifold has {} vertices and {} edges and {} faces and {}
// simplices.\n", manifold.N0(), manifold.N1(), manifold.N2(),
// manifold.N3());
}
catch (...)
{
std::cerr << "print_manifold() went wrong ...\n";
throw;
} // print_manifold
And:
SCENARIO("Printing results", "[utility]")
{
// redirect std::cout
stringstream buffer;
cout.rdbuf(buffer.rdbuf());
GIVEN("A Manifold3")
{
Manifold3 const manifold(640, 4);
WHEN("We want to print statistics on a manifold.")
{
THEN("Statistics are successfully printed.")
{
print_manifold(manifold);
CHECK_THAT(buffer.str(), Catch::Contains("Manifold has"));
}
}
}
Is there a way to capture the output generated by fmt::print going to stdout?
When I comment out the cout code and uncomment the fmt code, I get the buffer produced by previous instances of cout <<.
This is more of a C stdio than {fmt} question but you can redirect stdout to a pipe and read the output from it as described in the answers to Redirecting stdout to pipe in C. This is not a great unit test though because it depends on the global state but your current test has the same problem.

Processing all passed overloads at once

I'm tired of making up on the spot debug codes and including <iostream> in every single file. So I wanted to make myself a universal, self-contained and lightweight debug class, that I would just include in the header, and forget.
I want to use something along the lines of
#include "debug.hpp"
debug DBG;
DBG << "foo and" << " bar";
//Or even better, just include it and do debug() << "foo and" << " bar";
So, I wrote this:
#include <iostream>
#include <string>
#include <chrono>
#include <ctime>
class Debug
{
public:
Debug &operator<<(std::string arg_0)
{
auto tempTime = std::chrono::system_clock::to_time_t(
std::chrono::system_clock::now() );
auto timeString(ctime(&tempTime));
timeString = timeString.substr(timeString.find(':') - 2, 8);
std::cout << timeString << " >> " << arg_0 << '\n';
return *this;
}
};
But of course, this doesn't work because, as I've learned, every overload operator causes this function (is it still called a function?) to trigger separately. Creating:
hour:minute:second >> foo and
hour:minute:second >> bar
Any way I could pass everything at once after the first overload operator appears? Maybe as a stringstream? Also, I won't be only passing strings, but anything that I need, will this require me to manually create a separate overload function for every signle type that I may pass?
P.S: Cross-plaform solution is optional, but welcome (Currently developing on Linux)
You may return an other class to do the job, something like:
class Helper
{
public:
~Helper() { std::cout << "\n"; }
template<typename T>
friend Helper&& operator << (Helper&&h, const T& t) {
std::cout << t;
return std::move(h);
}
};
class Debug
{
public:
template<typename T>
friend Helper operator<<(Debug&, const T& t)
{
auto tempTime = std::chrono::system_clock::to_time_t(
std::chrono::system_clock::now() );
auto timeString{ctime(&tempTime)};
timeString = timeString.substr(timeString.find(':') - 2, 8);
std::cout << timeString << " >> " << t;
return Helper{};
}
};
Each time you call operator<<, your code prints the time stamp and \n. And that's the problem. To avoid that, you can print the time stamp in the constructor of Debug, and print \n in the destructor.
class Debug {
public:
Debug() {
auto tempTime = std::chrono::system_clock::to_time_t(
std::chrono::system_clock::now() );
std::string timeString(ctime(&tempTime));
timeString = timeString.substr(timeString.find(':') - 2, 8);
std::cout << timeString;
}
~Debug() {
std::cout << "\n";
}
Debug &operator<<(std::string arg_0) {
std::cout << " >> " << arg_0;
return *this;
}
};
In order to debug types other than string, you make operator<< a template:
template <typename T>
Debug &operator<<(T &&arg_0) {
std::cout << " >> " << std::forward<T>(arg_0);
return *this;
}
I see 2 design problems here:
You try to create stream-like object. It means that it doesn't know, when the line ends, until you send EOL to it. Without this information, it doesn't know when to add prefix to "your" line and print it. Consider the two following situation:
DBG << "foo and" << " bar";
and
DBG << "foo and";
... (a lot of code) ...
DBG << " bar";
They look exactly the same inside your Debug class, because:
DBG << "foo and" << " bar"; == (DBG.operator<<("foo and")).operator<<(" bar");
And this is the same as:
DBG.operator<<("foo and");
DBG.operator<<("bar");
So you have to decide how to define the end of the message you want to print (and when do you want to measure the time: At the beginning or at the end of the message?).
When do you want to flush your stream? You have to send std::endl or std::flush to std::cout to flush it. Sending "\n" does not flush std::cout (this is important difference between std::endl and "\n"). If you do not flush it, it may be printed several minutes/hours later (it will wait in a buffer). On the other hand frequent buffer flushing may be a performance killer in application producing large amount of text.
Try to define how your stream should behave when you send to it "\n", std::endl and std::flush (std::endl should be converted to "\n"+std::flush).
About other questions:
I would use simple template to "transfer" parameter of operator<<() to std::cout. It would allow to use your class for any type that can be printed by std::cout. To make things simpler you can define the operator<<() outside your class, eg.:
template<typename tParam>
Debug &operator<<(Debug& stream, tParam const & myParam)
{
...
return stream;
}

Logging function which uses operator <<

I'd like to write a function for logging which should be used like this:
log(__FILE__) << "My message containing integer: " << 123 << " and double: " << 1.2;
This should print the following line, add endl and flush immediately:
main.cpp: My message containing integer: 123 and double: 1.2
My (simplified) attempt for the implementation of the function:
class Writer
{
public:
template<typename T>
Writer & operator<<(T t)
{
cout << t << endl;
cout.flush();
return (*this);
}
};
Writer log(const char* fileName)
{
cout << fileName << ": ";
return Writer();
}
int main(int argc, const char *argv[])
{
log(__FILE__) << "My message containing integer: " << 123 << "and double: " << 1.2;
return 0;
}
My problem is that because of L-R associativity of the operator<< the output is:
main.cpp: My message containing integer:
123
and double:
1.2
Is there any way how to implement the function or is my requirement for its usage unrealizable?
Ideally I'd like to use plain C++03 (i.e. no C++11 features, boost and non-standard libraries).
L-R associativity is not related to your problem (if you talk about line breaks). The problem is because you use endl after each write. You don't need it (and if you do that, then you don't need flush, because endl already flushes the output).
The easy solution to your problem:
class Writer
{
public:
template<typename T>
Writer & operator<<(T t)
{
cout << t;
return (*this);
}
~Writer()
{
try {
cout << endl;
}
catch (...) {
// You have to make sure that no
// exception leaves destructor
}
}
};
It is also worth to notice, that your approach is not really scalable: it is impossible to use your code in multi-threaded environment. Assume that two threads are writing into your logging:
Thread 1: log(__FILE__) << "a" << "b" << "c";
Thread 2: log(__FILE__) << "a" << "b" << "c";
Here you can easily get a message "aabbcc\n\n" in your logfile, which is highly undesirable.
In order to avoid that, you can have a static mutex object inside log() function, which you pass into Writer constructor. Then you have to lock it in the constructor and unlock it in the destructor. It will guarantee the synchronization of concurrent writing of different entries.

multiple threads writing to std::cout or std::cerr

I have OpenMP threads that write to the console via cout and cerr. This of course is not safe, since output can be interleaved. I could do something like
#pragma omp critical(cerr)
{
cerr << "my variable: " << variable << endl;
}
It would be nicer if could replace cerr with a thread-safe version, similar to the approach explained in the valgrind DRD manual (http://valgrind.org/docs/manual/drd-manual.html#drd-manual.effective-use) which involves deriving a class from std::ostreambuf. Ideally in the end I would just replace cerr with my own threaded cerr, e.g. simply:
tcerr << "my variable: " << variable << endl;
Such a class could print to the console as soon as it encounters an "endl". I do not mind if lines from different threads are interleaved, but each line should come only from one thread.
I do not really understand how all this streaming in C++ works, it is too complicated. Has anybody such a class or can show me how to create such a class for that purpose?
As others pointed out, in C++11, std::cout is thread-safe.
However if you use it like
std::cout << 1 << 2 << 3;
with different threads, the output can still be interleaved, since every << is a new function call which can be preceeded by any function call on another thread.
To avoid interleaving without a #pragma omp critical - which would lock everything - you can do the following:
std::stringstream stream; // #include <sstream> for this
stream << 1 << 2 << 3;
std::cout << stream.str();
The three calls writing 123 to the stream are happening in only one thread to a local, non-shared object, therefore aren't affected by any other threads. Then, there is only one call to the shared output stream std::cout, where the order of items 123 is already fixed, therefore won't get messed up.
You can use an approach similar to a string builder. Create a non-template class that:
offers templated operator<< for insertion into this object
internally builds into a std::ostringstream
dumps the contents on destruction
Rough approach:
class AtomicWriter {
std::ostringstream st;
public:
template <typename T>
AtomicWriter& operator<<(T const& t) {
st << t;
return *this;
}
~AtomicWriter() {
std::string s = st.str();
std::cerr << s;
//fprintf(stderr,"%s", s.c_str());
// write(2,s.c_str(),s.size());
}
};
Use as:
AtomicWriter() << "my variable: " << variable << "\n";
Or in more complex scenarios:
{
AtomicWriter w;
w << "my variables:";
for (auto & v : vars) {
w << ' ' << v;
}
} // now it dumps
You will need to add more overloads if you want manipulators, you can use write better than fprintf for the atomic write in the destructor, or std::cerr, you can generalize so that the destination is passed to the constructor (std::ostream/file descriptor/FILE*),
I don't have enough reputation to post a comment, but I wanted to post my addition to the AtomicWriter class to support std::endl and allow for other streams to be used besides std::cout. Here it is:
class AtomicWriter {
std::ostringstream st;
std::ostream &stream;
public:
AtomicWriter(std::ostream &s=std::cout):stream(s) { }
template <typename T>
AtomicWriter& operator<<(T const& t) {
st << t;
return *this;
}
AtomicWriter& operator<<( std::ostream&(*f)(std::ostream&) ) {
st << f;
return *this;
}
~AtomicWriter() { stream << st.str(); }
};
Put the following code in header file atomic_stream_macro.h
#ifndef atomic_stream_macro_h
#define atomic_stream_macro_h
#include <mutex>
/************************************************************************/
/************************************************************************/
extern std::mutex print_mutex;
#define PRINT_MSG(out,msg) \
{ \
std::unique_lock<std::mutex> lock (print_mutex); \
\
out << __FILE__ << "(" << __LINE__ << ")" << ": " \
<< msg << std::endl; \
}
/************************************************************************/
/************************************************************************/
#endif
Now the macro can be used from a file as follows.
#include <atomic_stream_macro.h>
#include <iostream>
int foo (void)
{
PRINT_MSG (std::cout, "Some " << "Text " << "Here ");
}
Finally, in the main.cxx, declare the mutex.
#include <mutex>
std::mutex print_mutex;
int main (void)
{
// launch threads from here
}
You could do it by inheriting std::basic_streambuf, and override the correct functions to make it threadsafe. Then use this class for your stream objects.