incremental build - Page 248

boost::function & boost::lambda again

- by John Dibling

Follow-up to post: http://stackoverflow.com/questions/2978096/using-width-precision-specifiers-with-boostformat I'm trying to use boost::function to create a function that uses lambdas to format a string with boost::format. Ultimately what I'm trying to achieve is using width & precision specifiers for strings with format. boost::format does not support the use of the * width & precision specifiers, as indicated in the docs: Width or precision set to asterisk (*) are used by printf to read this field from an argument. e.g. printf("%1$d:%2$.*3$d:%4$.*3$d\n", hour, min, precision, sec); This class does not support this mechanism for now. so such precision or width fields are quietly ignored by the parsing. so I'm trying to find other ways to accomplish the same goal. Here is what I have so far, which isn't working: #include <string> #include <boost\function.hpp> #include <boost\lambda\lambda.hpp> #include <iostream> #include <boost\format.hpp> #include <iomanip> #include <boost\bind.hpp> int main() { using namespace boost::lambda; using namespace std; boost::function<std::string(int, std::string)> f = (boost::format("%s") % boost::io::group(setw(_1*2), setprecision(_2*2), _3)).str(); std::string s = (boost::format("%s") % f(15, "Hello")).str(); return 0; } This generates many compiler errors: 1>------ Build started: Project: hacks, Configuration: Debug x64 ------ 1>Compiling... 1>main.cpp 1>.\main.cpp(15) : error C2872: '_1' : ambiguous symbol 1> could be 'D:\Program Files (x86)\boost\boost_1_42\boost/lambda/core.hpp(69) : boost::lambda::placeholder1_type &boost::lambda::`anonymous-namespace'::_1' 1> or 'D:\Program Files (x86)\boost\boost_1_42\boost/bind/placeholders.hpp(43) : boost::arg<I> `anonymous-namespace'::_1' 1> with 1> [ 1> I=1 1> ] 1>.\main.cpp(15) : error C2664: 'std::setw' : cannot convert parameter 1 from 'boost::lambda::placeholder1_type' to 'std::streamsize' 1> No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called 1>.\main.cpp(15) : error C2872: '_2' : ambiguous symbol 1> could be 'D:\Program Files (x86)\boost\boost_1_42\boost/lambda/core.hpp(70) : boost::lambda::placeholder2_type &boost::lambda::`anonymous-namespace'::_2' 1> or 'D:\Program Files (x86)\boost\boost_1_42\boost/bind/placeholders.hpp(44) : boost::arg<I> `anonymous-namespace'::_2' 1> with 1> [ 1> I=2 1> ] 1>.\main.cpp(15) : error C2664: 'std::setprecision' : cannot convert parameter 1 from 'boost::lambda::placeholder2_type' to 'std::streamsize' 1> No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called 1>.\main.cpp(15) : error C2872: '_3' : ambiguous symbol 1> could be 'D:\Program Files (x86)\boost\boost_1_42\boost/lambda/core.hpp(71) : boost::lambda::placeholder3_type &boost::lambda::`anonymous-namespace'::_3' 1> or 'D:\Program Files (x86)\boost\boost_1_42\boost/bind/placeholders.hpp(45) : boost::arg<I> `anonymous-namespace'::_3' 1> with 1> [ 1> I=3 1> ] 1>.\main.cpp(15) : error C2660: 'boost::io::group' : function does not take 3 arguments 1>.\main.cpp(15) : error C2228: left of '.str' must have class/struct/union 1>Build log was saved at "file://c:\Users\john\Documents\Visual Studio 2005\Projects\hacks\x64\Debug\BuildLog.htm" 1>hacks - 7 error(s), 0 warning(s) ========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ========== My fundamental understanding of boost's lambdas and functions is probably lacking. How can I get this to work?

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