C++ string template library

Question

I want simple C++ string based template library to replace strings at runtime.

For example, I will use

string template = \"My name is {{name}}\";


        

Answer 1

If you have a function that replaces all occurrences of a string with another string:

std::string replace_all(std::string str, const std::string &remove, const std::string &insert) 
{
    std::string::size_type pos = 0;
    while ((pos = str.find(remove, pos)) != std::string::npos)
    {
        str.replace(pos, remove.size(), insert);
        pos++;
    }

    return str;
}

Then you can do this:

std::string pattern = "My name is {{first_name}} {{last_name}} and I live in {{location}}";

std::string str = replace_all(replace_all(replace_all(pattern, 
                       "{{first_name}}", "Homer"), 
                       "{{last_name}}", "Simpson"), 
                       "{{location}}", "Springfield");

Answer 2

You can have a look at Inja. It is a simple, header-only template engine and does what you're asking for. There you can just call

data["name"] = "world";
inja::render("Hello {{ name }}!", data); // Returns "Hello world!"

Answer 3

string skeleton = "My name is {{name}}";
string placeholder = "{{name}}";
string::size_type pos = skeleton.find(placeholder);
while( pos != string::npos ) {
    skeleton.replace(pos, placeholder.length(), "Gopalakrishnan");
    pos = skeleton.find(placeholder, ++pos);
}

Answer 4

If you are new to C++, adding new libraries (template or otherwise) to your installation will only increase the learning curve. This is something you can do simply, elegantly, and efficiently with the built-in features.

Unlike similar answers, this code makes only one pass over the input and scales well with large dictionaries:

// header
#include <map>
#include <sstream>

typedef std::map< std::string, std::string > subst_map;

// implementation
using namespace std;

string do_substitutions( string const &in, subst_map const &subst ) {
    ostringstream out;
    size_t pos = 0;
    for (;;) {
        size_t subst_pos = in.find( "{{", pos );
        size_t end_pos = in.find( "}}", subst_pos );
        if ( end_pos == string::npos ) break;

        out.write( &* in.begin() + pos, subst_pos - pos );

        subst_pos += strlen( "{{" );
        subst_map::const_iterator subst_it
            = subst.find( in.substr( subst_pos, end_pos - subst_pos ) );
        if ( subst_it == subst.end() ) throw runtime_error( "undefined substitution" );

        out << subst_it->second;
        pos = end_pos + strlen( "}}" );
    }
    out << in.substr( pos, string::npos );
    return out.str();
}

// usage
pair< string, string > substitutions_init[] = {
    make_pair( "firstname", "homer" ),
    make_pair( "lastname", "simpson" )
};
subst_map substitutions
    ( substitutions_init, substitutions_init + sizeof(substitutions_init)/sizeof(*substitutions_init) );

int main() {
    cerr << do_substitutions( "Mr. {{lastname}}, {{firstname}} esquire", substitutions ) << endl;
}