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Windows are the primary containers in Dear ImGui. They can contain any widgets, can be moved, resized, collapsed, and docked (in the docking branch).

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Begin

Starts a new window and pushes it onto the window stack. 
bool Begin(const char* name, bool* p_open = NULL, ImGuiWindowFlags flags = 0);

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Parameters

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name
const char*
required
Window title and unique identifier. Use ”###” to separate display name from ID (e.g., “MyWindow###UniqueID”).
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p_open
bool*
default:"NULL"
Pointer to a boolean controlling window visibility. When not NULL, displays a close button that sets this to false when clicked.
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flags
ImGuiWindowFlags
default:"0"
Window behavior flags. See ImGuiWindowFlags_ enum for available options.

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Returns

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return
bool
Returns false if the window is collapsed or fully clipped. You can use this to skip rendering content, but you must still call End().

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Description

Begin() pushes a window onto the stack and begins appending to it. Key points:
  • Always call End() for each Begin(), regardless of return value
  • Return value indicates visibility, not whether you should call End()
  • Multiple Begin()/End() pairs with the same name append to the same window
  • First call in a frame determines window flags and p_open behavior

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Example

// Simple window
ImGui::Begin("My Window");
ImGui::Text("Hello, World!");
ImGui::End();

// Window with close button
static bool show_window = true;
if (show_window) {
    ImGui::Begin("Closeable Window", &show_window);
    ImGui::Text("You can close this window.");
    ImGui::End();
}

// Skip content rendering when collapsed
if (ImGui::Begin("Optimized Window")) {
    // Only render expensive content when visible
    ImGui::Text("Visible content");
    for (int i = 0; i < 1000; i++) {
        ImGui::Text("Line %d", i);
    }
}
ImGui::End(); // Always call End()

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Common Flags

// No title bar
ImGui::Begin("Window", NULL, ImGuiWindowFlags_NoTitleBar);

// No resize grip
ImGui::Begin("Window", NULL, ImGuiWindowFlags_NoResize);

// No moving
ImGui::Begin("Window", NULL, ImGuiWindowFlags_NoMove);

// No scrollbars
ImGui::Begin("Window", NULL, ImGuiWindowFlags_NoScrollbar);

// Always auto-resize
ImGui::Begin("Window", NULL, ImGuiWindowFlags_AlwaysAutoResize);

// Menu bar
ImGui::Begin("Window", NULL, ImGuiWindowFlags_MenuBar);
if (ImGui::BeginMenuBar()) {
    if (ImGui::BeginMenu("File")) {
        ImGui::MenuItem("Open");
        ImGui::EndMenu();
    }
    ImGui::EndMenuBar();
}
ImGui::End();

// Combine multiple flags
ImGui::Begin("Fixed Window", NULL, 
    ImGuiWindowFlags_NoResize | 
    ImGuiWindowFlags_NoMove | 
    ImGuiWindowFlags_NoCollapse);
Important: Due to legacy reasons, Begin/End are inconsistent with other BeginXXX/EndXXX functions. You must always call End() even if Begin() returns false. This will be fixed in a future update.

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End

Pops the current window from the stack. 
void End();

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Description

Ends the current window context. Must be called for every Begin() call, regardless of its return value.

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Example

// Correct usage
if (ImGui::Begin("Window")) {
    ImGui::Text("Content");
}
ImGui::End(); // Always called

// Also correct
ImGui::Begin("Window");
ImGui::Text("Content");
ImGui::End();

// WRONG - Don't do this!
if (ImGui::Begin("Window")) {
    ImGui::Text("Content");
    ImGui::End(); // Only called when visible - INCORRECT!
}
The bottom of the window stack always contains a window called “Debug”. You cannot pop this window.

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Window Positioning and Sizing

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SetNextWindowPos

Sets the position of the next window before Begin(). 
void SetNextWindowPos(const ImVec2& pos, ImGuiCond cond = 0, const ImVec2& pivot = ImVec2(0, 0));

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Parameters

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pos
const ImVec2&
required
Window position in screen coordinates.
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cond
ImGuiCond
default:"0"
Condition for setting position. Use ImGuiCond_FirstUseEver to allow user to move window.
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pivot
const ImVec2&
default:"ImVec2(0, 0)"
Pivot point for positioning. (0,0) = top-left, (0.5,0.5) = center, (1,1) = bottom-right.

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Example

// Position at top-left corner
ImGui::SetNextWindowPos(ImVec2(0, 0));
ImGui::Begin("Corner Window");

// Center window on screen
ImGuiIO& io = ImGui::GetIO();
ImVec2 center = ImVec2(io.DisplaySize.x * 0.5f, io.DisplaySize.y * 0.5f);
ImGui::SetNextWindowPos(center, ImGuiCond_Always, ImVec2(0.5f, 0.5f));
ImGui::Begin("Centered Window");

// Position only on first use, then user can move it
ImGui::SetNextWindowPos(ImVec2(100, 100), ImGuiCond_FirstUseEver);
ImGui::Begin("User Moveable Window");

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SetNextWindowSize

Sets the size of the next window before Begin(). 
void SetNextWindowSize(const ImVec2& size, ImGuiCond cond = 0);

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Parameters

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size
const ImVec2&
required
Window size. Set axis to 0.0f to force auto-fit on that axis.
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cond
ImGuiCond
default:"0"
Condition for setting size.

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Example

// Fixed size window
ImGui::SetNextWindowSize(ImVec2(400, 300));
ImGui::Begin("Fixed Size");

// Auto-fit width, fixed height
ImGui::SetNextWindowSize(ImVec2(0, 300));
ImGui::Begin("Auto Width");

// Set size only once
ImGui::SetNextWindowSize(ImVec2(500, 400), ImGuiCond_FirstUseEver);
ImGui::Begin("User Resizable");

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Complete Window Example

struct AppState {
    bool show_main_window = true;
    bool show_settings = false;
    bool show_debug = false;
};

static AppState state;

void RenderUI() {
    // Main menu bar
    if (ImGui::BeginMainMenuBar()) {
        if (ImGui::BeginMenu("Windows")) {
            ImGui::MenuItem("Main Window", NULL, &state.show_main_window);
            ImGui::MenuItem("Settings", NULL, &state.show_settings);
            ImGui::MenuItem("Debug", NULL, &state.show_debug);
            ImGui::EndMenu();
        }
        ImGui::EndMainMenuBar();
    }
    
    // Main window
    if (state.show_main_window) {
        ImGui::SetNextWindowPos(ImVec2(50, 50), ImGuiCond_FirstUseEver);
        ImGui::SetNextWindowSize(ImVec2(500, 400), ImGuiCond_FirstUseEver);
        
        if (ImGui::Begin("Main Window", &state.show_main_window, ImGuiWindowFlags_MenuBar)) {
            if (ImGui::BeginMenuBar()) {
                if (ImGui::BeginMenu("File")) {
                    if (ImGui::MenuItem("Settings")) {
                        state.show_settings = true;
                    }
                    ImGui::EndMenu();
                }
                ImGui::EndMenuBar();
            }
            
            ImGui::Text("Application content goes here");
            ImGui::Separator();
            ImGui::Text("FPS: %.1f", ImGui::GetIO().Framerate);
        }
        ImGui::End();
    }
    
    // Settings window
    if (state.show_settings) {
        ImGui::SetNextWindowSize(ImVec2(300, 200), ImGuiCond_FirstUseEver);
        if (ImGui::Begin("Settings", &state.show_settings)) {
            ImGui::Text("Application Settings");
            // Settings controls here
        }
        ImGui::End();
    }
    
    // Debug window (fixed position, no resize)
    if (state.show_debug) {
        ImGui::SetNextWindowPos(ImVec2(10, 10), ImGuiCond_Always);
        ImGui::SetNextWindowSize(ImVec2(300, 100), ImGuiCond_Always);
        if (ImGui::Begin("Debug", &state.show_debug, 
            ImGuiWindowFlags_NoResize | ImGuiWindowFlags_NoMove)) {
            ImGui::Text("Debug Information");
            ImGui::Text("Frame time: %.3f ms", 1000.0f / ImGui::GetIO().Framerate);
        }
        ImGui::End();
    }
}

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