C/C++教學

C/C++ snippets 程式碼片段

以下紀錄我常用的程式碼片段

除錯類

請看這篇文章

自己的

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void readFromFile(const char *filename)
{
    char buf[128];
    FILE *fp = NULL;
    int ret;
    float num[3];

    fp = fopen(filename, "r");
    if (fp == NULL) {
        printf("Error: can not read %s file!\n", filename);
        return;
    }

    while (1) {
        ret = fscanf(fp, "%f, %f, %f", &num[0], &num[1], &num[2]);
        if (ret == EOF)
            break;
    }

    if (fp != NULL)
        fclose(fp);
}

別人的

http://www.roxlu.com/2013/014/c—file-operation-snippets

Open a file in binary mode

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std::ifstream ifs;
ifs.open("input.bin", std::ios::in | std::ios::binary);
if (!ifs.is_open()) {
    printf("Error trying to open the file.\n");
    ::exit(EXIT_FAILURE);
}

讀取二進制檔到現存的 std::vector (Reading a binary file into an existing std::vector)

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std::ifstream ifs;
ifs.open("input.bin", std::ios::in | std::ios::binary);

std::vector<char> buffer;
buffer.assign(std::istreambuf_iterator<char>(ifs), std::istreambuf_iterator<char>());

ifs.close();

讀取文字檔到 buf 裡 (Reading a text file into a string (note the extra parathesis for std::istreambuf_iterator))

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std::ifstream ifs(filepath.c_str(), std::ios::in);
if(!ifs.is_open()) {
    return false;
}

std::string str( (std::istreambuf_iterator<char>(ifs)) , std::istreambuf_iterator<char>());

Reading a text file using pure C

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int read_file(const char* path, char* buf, int len) {
    /* try to open the file */
    FILE* fp = fopen(path, "r");
    if (!fp) { 
        printf("Error: cannot read file: %s\n", path);
        return -1;
    }

    /* find size */
    long size = 0;
    fseek(fp, 0, SEEK_END);
    size = ftell(fp);
    fseek(fp, 0, SEEK_SET);

    if (size > len) {
        printf("Error: file size it too large for given buffer. We need: %ld bytes.\n", size);
        fclose(fp);
        return -2;
    }

    size_t r = fread(buf, size, 1, fp);
    if (r != 1) {
        printf("Error: cannot read file into buffer.\n");
        fclose(fp);
        return -3;
    }
  
    return (int)size;
}

檔案大小 (Check the file size)

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ifs.seekg(0, std::ifstream::end);
size_t fsize = ifs.tellg();
printf("File size: %ld.\n", fsize);

跳至某位置 (Jump to a position)

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ifs.seekg(1024, std::ios_base::beg);     
if (!ifs) {
    printf("Error trying to jump to a position.\n");
    ::exit(EXIT_FAILURE);
}

取得現在位置 (Get the curent position)

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size_t pos = ifs.tellg();
printf("Current position: %ld.\n", pos);

Jump to a absolute position

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ifs.seekg(0, std::ios_base::beg);

Read some bytes

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const size_t buffer_size = 1024 * 1024 * 10;
char* buffer = new char[buffer_size];

size_t read_size = buffer_size;
if(fsize < read_size) {
    read_size = fsize;
    printf("The file is smaller then the number buffer.\n");
}

printf("Reading: %ld\n", read_size);
ifs.read(buffer, read_size);
if(!ifs) {
    printf("Error trying to into buffer.\n");
    exit(EXIT_FAILURE);
}

if(ifs.gcount() != sizeof(buffer)) {
    printf("Number of bytes is not the same as requested.\n");
}
printf("Read: %ld bytes into buffer\n", ifs.gcount());

delete[] buffer;
buffer = NULL;

讀取二進制檔 (Read all bytes)

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do {
    ifs.read((char*)buffer, sizeof(buffer));
    nread = ifs.gcount();
    for (size_t i = 0; i < nread; ++i) {
        sprintf(hexbuf, "0x%02x", buffer[i]);
        ofs_source << hexbuf << ", ";
        ++nchars_written;
        if (nchars_written >= 16) {
            ofs_source << "\n  ";
            nchars_written = 0;
        }
    }
} while (ifs.good());

寫入二進制檔 (Write some bytes)

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std::vector<char> buffer;
buffer.push_back('a');
buffer.push_back('b');
buffer.push_back('c');

std::ofstream ofs("output.bin", std::ios::out | std::ios::binary);
ofs.write(&buffer.front(), buffer.size());
ofs.close();

Write the contents of a std::stringstream to a file

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std::ofstream ofs("output.txt", std::ios::out);
if(!ofs.is_open()) { 
    return;
}
 
std::stringstream ss;
ss << "some content.";

ofs << ss.rdbuf();

ofs.close();

And the complete source:

Compile with:

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g++ main.cpp -g -o out && ./out

Complete source

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#include <iostream>
#include <fstream>
#include <stdio.h>

int main() {

    // open a file in binary mode
    std::ifstream ifs;
    ifs.open("test.mov", std::ios::in | std::ios::binary);
    if (!ifs.is_open()) {
        printf("Error trying to open the file.\n");
        ::exit(EXIT_FAILURE);
    }

    // check the file size
    ifs.seekg(0, std::ifstream::end);
    size_t fsize = ifs.tellg();
    printf("File size: %ld.\n", fsize);


    // jump to a position from the start of the file
    ifs.seekg(1024, std::ios_base::beg);
    if (!ifs) {
        printf("Error trying to jump to a position.\n");
        ::exit(EXIT_FAILURE);
    }

    // get current position
    size_t pos = ifs.tellg();
    printf("Current position: %ld.\n", pos);


    // go back to the beginning of the file.
    ifs.seekg(0, std::ios_base::beg);

    // read some bytes
    const size_t buffer_size = 1024 * 1024 * 10;
    char* buffer = new char[buffer_size];

    size_t read_size = buffer_size;
    if (fsize < read_size) {
        read_size = fsize;
        printf("The file is smaller then the number buffer.\n");
    }

    printf("Reading: %ld\n", read_size);
    ifs.read(buffer, read_size);
    if (!ifs) {
        printf("Error trying to into buffer.\n");
        exit(EXIT_FAILURE);
    }

    if (ifs.gcount() != sizeof(buffer)) {
        printf("Number of bytes is not the same as requested.\n");
    }
    printf("Read: %ld bytes into buffer\n", ifs.gcount());

    delete[] buffer;
    buffer = NULL;
    return 0;
}

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