итоговый вариант

main.cpp

@@ -1,9 +1,11 @@
 #include <iostream>
+#include <sstream>
+#include <string>
 #include <vector>
+#include <curl/curl.h>
 #include "histogram.h"
 #include "text.h"
 #include "svg.h"
-#include <curl/curl.h>
 
 using namespace std;
 
@@ -32,35 +34,52 @@ Input input_data(istream& in, bool prompt) {
     return input;
 }
 
+size_t write_data(void* items, size_t item_size, size_t item_count, void* ctx) {
+    size_t data_size = item_size * item_count;
+
+    stringstream* buffer = reinterpret_cast<stringstream*>(ctx);
+
+    buffer->write(reinterpret_cast<const char*>(items), data_size);
 
-size_t write_data(void* ptr, size_t size, size_t nmemb, void* stream) {
-    size_t written = fwrite(ptr, size, nmemb, stdout);
-    return written;
+    return data_size;
 }
 
-int main(int argc, char* argv[]) {
-    if (argc > 1) {
-        CURL* curl = curl_easy_init();
-        if (curl) {
-            curl_easy_setopt(curl, CURLOPT_URL, argv[1]);
-            curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, write_data);
+Input download(const string& address) {
+    stringstream buffer;
 
+    CURL* curl = curl_easy_init();
+    if (curl) {
+        curl_easy_setopt(curl, CURLOPT_URL, address.c_str());
 
-            CURLcode res = curl_easy_perform(curl);
-            if (res != CURLE_OK) {
-                cerr << "cURL error: " << curl_easy_strerror(res) << endl;
-            }
+        curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, write_data);
+        curl_easy_setopt(curl, CURLOPT_WRITEDATA, &buffer);
 
+        CURLcode res = curl_easy_perform(curl);
+        if (res != CURLE_OK) {
+            cerr << "cURL error: " << curl_easy_strerror(res) << endl;
             curl_easy_cleanup(curl);
-        } else {
-            cerr << "curl_easy_init() failed" << endl;
+            exit(1);
         }
-        return 0;
+
+        curl_easy_cleanup(curl);
+    } else {
+        cerr << "curl_easy_init() failed" << endl;
+        exit(1);
+    }
+
+    return input_data(buffer, false);
+}
+
+int main(int argc, char* argv[]) {
+    Input input;
+
+    if (argc > 1) {
+        input = download(argv[1]);
+    } else {
+        input = input_data(cin, true);
     }
 
-    curl_global_init(CURL_GLOBAL_ALL);
-    auto in = input_data(cin, true);
-    auto bins = make_histogram(in.numbers, in.bin_count);
+    const auto bins = make_histogram(input.numbers, input.bin_count);
     show_histogram_svg(bins);
 
     return 0;