How to check if string ends with .txt

Unfortunately this useful function is not in the standard library. It is easy to write.

bool has_suffix(const std::string &str, const std::string &suffix)
{
    return str.size() >= suffix.size() &&
           str.compare(str.size() - suffix.size(), suffix.size(), suffix) == 0;
}

Use std::string::substr

if (filename.substr(std::max(4, filename.size())-4) == std::string(".txt")) {
    // Your code here
}

Using boost ends_with predicate:

#include <boost/algorithm/string/predicate.hpp>

if (boost::ends_with(fileName, ".txt")) { /* ... */ }

You've gotten quite a few answers already, but I decided to add yet another:

bool ends_with(std::string const &a, std::string const &b) {
    auto len = b.length();
    auto pos = a.length() - len;
    if (pos < 0)
        return false;
    auto pos_a = &a[pos];
    auto pos_b = &b[0];
    while (*pos_a)
        if (*pos_a++ != *pos_b++)
            return false;
    return true;
}

Since you have gotten quite a few answers, perhaps a quick test and summary of results would be worthwhile:

#include <iostream>
#include <string>
#include <vector>
#include <time.h>
#include <iomanip>

bool ends_with(std::string const &a, std::string const &b) {
    auto len = b.length();
    auto pos = a.length() - len;
    if (pos < 0)
        return false;
    auto pos_a = &a[pos];
    auto pos_b = &b[0];
    while (*pos_a)
        if (*pos_a++ != *pos_b++)
            return false;
    return true;
}

bool ends_with_string(std::string const& str, std::string const& what) {
    return what.size() <= str.size()
        && str.find(what, str.size() - what.size()) != str.npos;
}

bool has_suffix(const std::string &str, const std::string &suffix)
{
    return str.size() >= suffix.size() &&
        str.compare(str.size() - suffix.size(), suffix.size(), suffix) == 0;
}

bool has_suffix2(const std::string &str, const std::string &suffix)
{
    bool index = str.find(suffix, str.size() - suffix.size());
    return (index != -1);
}

bool isEndsWith(const std::string& pstr, const std::string& substr)
{
    int tlen = pstr.length();
    int slen = substr.length();

    if (slen > tlen)
        return false;

    const char* tdta = pstr.c_str();
    const char* sdta = substr.c_str();

    while (slen)
    {
        if (tdta[tlen] != sdta[slen])
            return false;

        --slen; --tlen;
    }
    return true;
}

bool ends_with_6502(const std::string& str, const std::string& end) {
    size_t slen = str.size(), elen = end.size();
    if (slen <= elen) return false;
    while (elen) {
        if (str[--slen] != end[--elen]) return false;
    }
    return true;
}

bool ends_with_rajenpandit(std::string const &file, std::string const &suffix) {
    int pos = file.find(suffix);
    return (pos != std::string::npos);
}

template <class F>
bool test(std::string const &label, F f) {
    static const std::vector<std::pair<std::string, bool>> tests{
        { "this is some text", false },
        { "name.txt.other", false },
        { "name.txt", true }
    };
    bool result = true;

    std::cout << "Testing: " << std::left << std::setw(20) << label;
    for (auto const &s : tests)
        result &= (f(s.first, ".txt") == s.second);
    if (!result) {
        std::cout << "Failed\n";
        return false;
    }
    clock_t start = clock();
    for (int i = 0; i < 10000000; i++)
        for (auto const &s : tests)
            result &= (f(s.first, ".txt") == s.second);
    clock_t stop = clock();
    std::cout << double(stop - start) / CLOCKS_PER_SEC << " Seconds\n";
    return result;
}

int main() {
    test("Jerry Coffin", ends_with);
    test("Dietrich Epp", has_suffix);
    test("Dietmar", ends_with_string);
    test("Roman", isEndsWith);
    test("6502", ends_with_6502);
    test("rajenpandit", ends_with_rajenpandit);
}

Results with gcc:

Testing: Jerry Coffin           3.416 Seconds
Testing: Dietrich Epp           3.461 Seconds
Testing: Dietmar                3.695 Seconds
Testing: Roman                  3.333 Seconds
Testing: 6502                   3.304 Seconds
Testing: rajenpandit            Failed

Results with VC++:

Testing: Jerry Coffin           0.718 Seconds
Testing: Dietrich Epp           0.982 Seconds
Testing: Dietmar                1.087 Seconds
Testing: Roman                  0.883 Seconds
Testing: 6502                   0.927 Seconds
Testing: rajenpandit            Failed

Yes, those were run on identical hardware, and yes I ran them a number of times, and tried different optimization options with g++ to see if I could get it to at least come sort of close to matching VC++. I couldn't. I don't have an immediate explanation of why g++ produces so much worse code for this test, but I'm fairly confident that it does.